feat(sdd): add configurable search strategy for code-reading phases

[Lotto724]

Summary

• Add search_strategy config (grep | hybrid) to SDD skill system, enabling MCP RAG-based semantic code search with grep fallback
• New Section E in sdd-phase-common.md defines the cascade protocol: RAG → Grep → targeted Read
• Step 3.5 in sdd-init auto-detects MCP RAG tools (same pattern as strict_tdd — no interactive prompts)
• Optional reindex_tool fires once per sdd-apply batch (fire-and-forget)
• Backwards compatible: missing config = grep mode = current behavior unchanged

Closes #266

Changes

File	What changed
_shared/sdd-phase-common.md	Section E: Code Search Protocol (cascade, mode branching, reindex hook, fallback)
_shared/persistence-contract.md	search_strategy schema in sub-agent context
sdd-init/SKILL.md	Step 3.5 (MCP auto-detection) + config output + 3 summary variants
sdd-explore/SKILL.md	Step 3 reads config + references Section E
sdd-apply/SKILL.md	Step 2 reads+caches config via Section E + Step 6 reindex hook
sdd-verify/SKILL.md	Step 4 reads config + references Section E
testdata/golden/sdd-*-init.golden	Regenerated (7 files)

Design decisions

• Two modes only (grep / hybrid) — no standalone rag mode (too risky without fallback)
• Follows strict_tdd pattern: detect in sdd-init → persist → phases branch on config
• MCP-agnostic: stores tool name, not product — works with any RAG MCP server
• Reindex is optional: fire-and-forget after batch, never per-file; failure is non-fatal

Test plan

• 37/37 Go test packages passing (golden files regenerated)
• SDD verify: 20/20 spec scenarios compliant
• Judgment Day: 3 rounds, both judges APPROVED
• Manual: run sdd-init without RAG MCP server → confirm grep default, zero change
• Manual: run sdd-init with RAG MCP server → confirm hybrid auto-detected
• Manual: run sdd-explore in hybrid mode → confirm RAG → grep fallback cascade

[Alan-TheGentleman]

Review: feat(sdd): add configurable search strategy for code-reading phases

Nice work on the overall design — the pattern reuse from strict_tdd, backwards compatibility, and graceful degradation are all solid. One critical gap before this can merge:

Critical: Missing orchestrator forwarding

The search_strategy config is detected in sdd-init and consumed by sdd-explore/sdd-apply/sdd-verify, but the orchestrator instruction files (internal/assets/*/sdd-orchestrator.md) are not updated. There is no "Search Strategy Forwarding" section analogous to the existing "Strict TDD Forwarding (MANDATORY)" block.

Without this, the orchestrator will never pass search_strategy to sub-agents — they start with a fresh context and can't discover the config on their own.

Fix: Add a forwarding section to all 7 engine orchestrator files, mirroring the strict TDD forwarding pattern. Search for sdd-init/{project}, extract search_strategy, inject it into prompts for the 3 code-reading phases.

Minor suggestions

1. sdd-apply Step 2 item 3: Dense run-on sentence. Consider splitting into 3a (read search_strategy + cache) and 3b (read existing code via Section E).

2. Reindex hook placement: Sits between Step 6 and Step 7 without a step number. A "Step 6b" label would be more consistent.

3. Auto-detection heuristic: Consider noting that the tool description should mention "semantic"/"vector"/"embedding" to avoid false positives on generic code_search tools.

4. Section E ">=2 relevant snippets": Threshold is subjective — consider clarifying what "relevant" means or dropping the number.

The critical fix is mechanical (7 orchestrator files + goldens), then this is good to go.

[Lotto724]

Updated the PR to address all review feedback:\n- Added mandatory search_strategy forwarding in all orchestrator instructions (mirrors strict TDD forwarding).\n- Clarified sdd-apply Step 2 (split 3a/3b) and labeled reindex as Step 6b.

• Tightened auto-detection heuristic for RAG tools.
• Clarified hybrid search sufficiency criteria.
• Regenerated all SDD golden files.

Ready for re-review.