feat(tooling): add opt-in portable agents sync

[soulcodex]

Summary

This PR delivers issue #118 as an opt-in Agents Orchestration Switching capability via .agentic/agents.yaml, and finalizes the implementation in a forward-only model.

Final contract (v1)

• version: "1"
• enabled: <bool>
• agents: { <agent_name>: { description, prompt, providers{...} } }
• subagents: { <subagent_name>: { description, prompt, parent, providers{...} } }

What changed

• Added/validated orchestration contract and runtime flow:
  • schemas/agents.schema.json
  • tooling/lib/agents.sh
  • tooling/lib/vendors/codex/agents.sh
  • tooling/lib/vendors/opencode/agents.sh
• Codex provider-local agents path aligned to:
  • .codex/agents
• Removed backward-compat/migration approaches from this implementation:
  • removed active_vendor migration/fallback handling
  • removed .agentic/vendor-stash migration handling
  • removed sync fallback to library_path
  • removed compose/profile-level mcp: fallback
  • MCP seeding is now sourced from .agentic/mcp.yaml only
• Updated tests to match forward-only behavior and MCP source-of-truth:
  • tooling/lib/test.sh
• Updated docs to match current behavior and improve user-facing clarity:
  • docs/agents.md
  • docs/commands.md
  • docs/custom-rules.md
  • docs/customization.md
  • docs/vendors.md
  • vendors/claude/README.md
  • vendors/opencode/README.md

Validation

• just lint ✅
• just test ✅ (534/534)

Closes #118

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 75.18657% with 133 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tooling/lib/agents.sh	67.15%	45 Missing ⚠️
tooling/lib/vendors/codex/agents.sh	61.29%	36 Missing ⚠️
tooling/lib/vendors/opencode/agents.sh	61.29%	36 Missing ⚠️
tooling/lib/test.sh	94.50%	10 Missing ⚠️
tooling/lib/vendors/codex/switch.sh	75.00%	2 Missing ⚠️
tooling/lib/vendors/opencode/switch.sh	75.00%	2 Missing ⚠️
tooling/lib/mcp.sh	0.00%	1 Missing ⚠️
tooling/lib/vendor-switch.sh	80.00%	1 Missing ⚠️

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