feat(integrations): add Pi subagent support

[HerbertGao]

What does this PR do?

Adds Pi custom Subagent support to the canonical tool registry, converter, installer, docs, and existing tool-consistency CI.

• adds a dual-scope pi-agent-md target
• preserves Claude name, description, color, emoji, vibe, and persona body fields
• maps current Agency tools (Read/Write/Edit/Bash/Grep/Glob) to Pi built-ins
• stores original and unmapped tools under x-agency-*
• sets extensions: false for explicit tool lists
• installs globally to ~/.pi/agent/agents/ or to a supplied project .pi/agents/ path
• honors PI_CODING_AGENT_DIR and PI_AGENTS_DIR
• backs up divergent regular files and skips symlink/non-file targets

Generated integration output remains ignored. Pi requires @tintinweb/pi-subagents.

Agent Information

N/A — this PR does not add or modify an agent persona.

Validation performed

• ./scripts/check-tools.sh
• python3 scripts/check-hermes-plugin.py
• bash scripts/test-convert-frontmatter.sh
• bash scripts/test-agent-selection.sh
• generated and parsed all 273 Pi definitions with the actual pi-subagents parser
• companion App parity test matched all 273 generated files byte-for-byte

[HerbertGao]

Companion desktop App support: msitarzewski/agency-agents-app#88