feat(serve): expose the approved read operations over MCP

[e54-bot]

Summary

Closes #550. The six approved read-only operations join the MCP tool set — rules, cfg, findings, lintRules, persistentObjects, targetMetadata — taking tools/list from 15 to 21 tools. Every op was already implemented in ToolService and advertised by capabilities.operations; the change is the mcp flag per spec plus the contract/ADR documentation.

The workflow evidence in #550 was concrete: wright_references reports rule ids no exposed tool could resolve to a file, per-rule lint metadata (knownLimits, evidence, rationale) was unreachable for judging a finding's weight, and cfg had no MCP path for "which block waits" questions.

compile and capabilities stay unexposed per the issue's own scoping (capabilities duplicates tools/list; compile is a CLI convenience).

Changes

• tooldefs.rs: mcp: true on the six specs; descriptions and schemas were already in place.
• mcp.rs test: tool set now 21 names.
• agent-contract.md: tool table updated to the current set.
• ADR-0020: set documentation refreshed (it had already drifted — analyze, inspect, and the provider* edit ops were missing); MCP tool set omits read operations real agent workflows reach for #550's join is recorded with its evidence.

Verification

• tools/list on a real 205-file OPY project returns 21 tools.
• All six new tools called end-to-end over wright serve --transport mcp on Bastion: wright_rules (ids + spans), wright_cfg (kind-filtered blocks), wright_findings, wright_lint_rules (metadata + effective config), wright_persistent_objects, wright_target_metadata — all return real payloads, no isError.
• cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace --all-targets, git diff --check: clean.

Test plan

• tools/list advertises the six new tools; hidden ops (compile, capabilities) stay absent
• Each new tool answers correctly on a real project
• CI