feat(mcp): explain_rule tool

[aram-devdocs]

Summary

Add an explain_rule tool accepting rule_id. Reads
docs/src/rules/<slug>.md and returns the markdown + a
structuredContent block with severity, default config, and
examples.

Crate / area

plumb-mcp — rmcp-based stdio MCP server

Phase / batch

Part of #34 → Batch 3A (Tool scaffolding — read-side tools first).

Effort: S

PRD references

• PRD §14.2

Acceptance criteria

• Tool registered in tools/list; protocol test case in mcp_stdio.rs.
• Unknown rule_id returns a typed JSON-RPC error (-32602).
• Response cap ≤ 10 KB structured content.

Dependencies

• None (first in batch; batch-level deps gate this).

Review gates

• 02-spec-reviewer
• 03-code-quality-reviewer
• 05-architecture-validator
• 04-test-runner
• 06-security-auditor

How to deliver

Dispatch /gh-issue <this-issue-number>. See the root AGENTS.md and the scoped plumb-mcp/AGENTS.md for the invariants that apply.

Suggested delivery: gh-issue