feat: add nested-glob target type for monorepo/multi-module AGENTS.md discovery

[Copilot]

AgentSync only supported a single flat AGENTS.md source per agent config, making it unusable for monorepos and multi-module projects where subdirectories carry domain-specific agent instructions.

New nested-glob target type

Recursively discovers files matching a glob pattern under a search root and creates a symlink for each match using a destination template.

[agents.claude.targets.nested]
source  = "."                             # search root (relative to project root)
pattern = "**/AGENTS.md"                  # recursive glob; ** crosses directory boundaries
exclude = [".agents/**", "node_modules/**", "**/target/**", "**/build/**"]
destination = "{relative_path}/CLAUDE.md" # template expanded per discovered file
type    = "nested-glob"

Given clients/agent-runtime/AGENTS.md and modules/core-kmp/AGENTS.md, this produces:

• clients/agent-runtime/CLAUDE.md → symlink
• modules/core-kmp/CLAUDE.md → symlink

Destination template placeholders

Placeholder	Example	Notes
{relative_path}	clients/agent-runtime	parent dir relative to source; . at search root
{file_name}	AGENTS.md
{stem}	AGENTS
{ext}	md

Implementation details

• config.rs: new SyncType::NestedGlob variant; new exclude: Vec<String> field on TargetConfig; all_gitignore_entries() skips NestedGlob destinations (they are templates, not literal paths)
• linker.rs: process_nested_glob() walks with walkdir (follow_links = false); matches_path_glob() / path_glob_match() implement ** multi-segment glob matching on top of the existing single-segment matches_pattern(); expand_destination_template() handles placeholder substitution; clean() extended to re-discover and remove nested-glob symlinks
• 13 new unit tests covering discovery, exclusion, dry-run, clean, and missing search root

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Support for Nested AGENTS.md Files</issue_title>
<issue_description># Feature Request: Support for Nested AGENTS.md Files

Problem Statement

Currently, AgentSync only supports a single AGENTS.md source file in .agents/.
There is no mechanism to discover or sync AGENTS.md files that exist in
subdirectories of a project.

This limits AgentSync's usefulness in monorepos and multi-module projects
where different directories may need domain-specific agent instructions.

Use Cases

1. Monorepo with Domain-Specific Instructions

project-root/
├── .agents/
│   ├── AGENTS.md          # Root instructions (general)
│   └── agentsync.toml
├── clients/
│   └── agent-runtime/
│       └── AGENTS.md      # Rust-specific instructions
├── web/
│   └── apps/
│       └── dashboard/
│           └── AGENTS.md  # Vue/TypeScript-specific instructions
└── modules/
    └── core-kmp/
        └── AGENTS.md      # Kotlin-specific instructions

Each submodule has specialized instructions (Rust patterns, Vue conventions,
Kotlin patterns) that differ from the root. Currently, these nested
AGENTS.md files must be maintained manually or through external scripts.

2. Gradle Multi-Module Projects

android-app/
├── .agents/
│   └── AGENTS.md          # Android/Kotlin conventions
├── app/
│   └── AGENTS.md          # App-specific patterns
├── feature/
│   └── auth/
│       └── AGENTS.md      # Auth module patterns
└── core/
    └── AGENTS.md          # Core library patterns

3. Selective Inheritance

Many AI coding tools (Claude Code, OpenCode, etc.) already support reading
AGENTS.md from subdirectories with inheritance/cascading behavior —
the closest AGENTS.md to the current working directory takes precedence.

AgentSync should complement this by:

1. Discovering nested AGENTS.md files automatically
2. Syncing them to each agent's expected location

Proposed Solutions

Option A: Glob-Based Discovery (Recommended)

Add support for glob patterns in agentsync.toml:

[agents.claude]
enabled = true

# Existing: single file
[agents.claude.targets.instructions]
source = "AGENTS.md"
destination = "CLAUDE.md"
type = "symlink"

# New: nested discovery
[agents.claude.targets.nested]
enabled = true
source_dir = "."          # Root of glob search
pattern = "**/AGENTS.md"   # Glob pattern
destination_template = "{relative_path}/AGENTS.md"
type = "symlink"

This would:

• Discover all AGENTS.md files matching the pattern
• Create symlinks maintaining the relative directory structure
• For example: clients/agent-runtime/AGENTS.md → clients/agent-runtime/AGENTS.md (symlink)

Option B: Explicit Nested Targets

Keep the current flat structure but allow explicit per-directory targets:

[agents.claude]
enabled = true

[agents.claude.targets.instructions]
source = "AGENTS.md"
destination = "CLAUDE.md"
type = "symlink"

# Explicit nested targets
[agents.claude.targets.nested.agent-runtime]
source = "clients/agent-runtime/AGENTS.md"
destination = "clients/agent-runtime/AGENTS.md"
type = "symlink"

Downside: Requires manual configuration for each nested file.

Option C: Contextual Inheritance (Most Complex)

Implement a discovery mechanism that:

1. Scans for all **/AGENTS.md files
2. Creates a hierarchical symlink structure
3. Respects inheritance rules per agent

Downside: Significant complexity; may conflict with agent-specific inheritance behavior.

Technical Considerations

1. Circular Reference Prevention

Ensure glob patterns don't accidentally include the root .agents/AGENTS.md creating loops.

2. Exclusion Patterns

Some directories should be excluded:

• node_modules/
• target/ (Rust)
• build/ (Gradle)
• .git/
• dist/, coverage/

3. Incremental Sync

On subsequent runs, only sync changed/new/deleted files, not the entire glob result every time.

4. Conflict Resolution

If a nested AGENTS.md has the same content as the root, should it still create a symlink?
(Probably yes, for explicit submodule ownership.)

5. Performance

For large monorepos, glob patterns could match hundreds of files. Consider:

• Caching the glob result
• Parallel symlink creation
• Incremental updates

Example Configuration (Option A)

# agentsync.toml

[agents.claude]
enabled = true

[agents.claude.targets.instructions]
source = "AGENTS.md"
destination = "CLAUDE.md"
type = "symlink"

# Nested discovery with exclusions
[agents.claude.targets.nested]
enabled = true
source_dir = "."
pattern = "**/AGENTS.md"
exclude = [
    ".agents/**",
    "node_modules/**",
    "**/target/**",
    "**/build/**",
    "**/dist/**",
    "**/.git/**"
]
destination_template = "{relative_path}/CLAUDE.md"
type = "symlink"

[agents.codex]
enabled = true

# Similar nested config for Codex
[agents.codex.targets.nested]
enabled = true
source_dir = "."
pattern = "**/AG...

</details>



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- Fixes dallay/agentsync#233

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