feat(mcp): discoverability — --list-tools, --probe, rivet docs mcp

[avrabe]

Why

Scenario C dogfood (an AI integrator wiring rivet's MCP server into a
custom client) burned ~30 minutes on two unrelated discoverability
gaps: figuring out the JSON-RPC framing the server expects (line-delimited
vs LSP-style Content-Length) and writing throwaway JSON-RPC by hand to
enumerate the tool catalog. Both should be solvable in seconds.

What

Two changes on top of feat/v0.5.0-readme-quickstart-changelog:

1. rivet mcp --list-tools and rivet mcp --probe — two new flags
   on the existing rivet mcp subcommand. Neither starts a long-running
   server.

   • --list-tools walks the registered ToolRouter and prints the
     15-tool catalog. Default output is a human table; --format json
     emits the JSON-RPC tools/list payload exactly as the wire server
     would. Works without a project loaded — useful as a "is this
     binary an MCP-capable rivet?" check before any setup.
   • --probe runs the in-process equivalent of tools/call rivet_list
     (no args) against the current project and prints the decoded
     result.content[0].text payload. Verifies the project loads, the
     schema parses, and a real read tool returns artifacts — without
     spinning up stdio.

   Both reuse the exact handlers the wire server dispatches to, so
   their output cannot drift from a real session.

2. rivet docs mcp — a new embedded doc topic (~1400 words) that
   covers what the server exposes, the wire format (line-delimited
   JSON-RPC, NOT LSP Content-Length), the 3-message handshake (with the
   easily-forgotten notifications/initialized notification), the
   15-tool catalog with inputs, the result.content[0].text
   double-parse gotcha, three smoke-test recipes (one of them the
   bash-only raw JSON-RPC fallback), the mutate-then-rivet_reload
   convention, and a pointer to the upstream MCP spec. Registered in
   the docs index so rivet docs lists mcp alongside cli,
   cross-repo, etc.

   Also amends rivet docs cli to mention the new flags and cross-link
   to rivet docs mcp.

Test plan

• cargo check --workspace — clean
• cargo clippy --workspace --all-targets -- -D warnings — clean
  (only pre-existing MSRV info warning, no new lint errors)
• cargo fmt --all
• cargo test -p rivet-cli --test mcp_integration — 22/22 pass,
  including test_tools_list_returns_all_15_tools
• rivet docs lists mcp under Reference
• rivet docs mcp prints the new topic (1411 words)
• rivet mcp --list-tools lists 15 tools with parameters
• rivet mcp --list-tools --format json emits a JSON-RPC
  tools/list-shaped payload (result.tools[] with name,
  description, inputSchema)
• rivet mcp --probe returns the rivet_list payload (759
  artifacts on this project) without errors
• rivet --project /tmp mcp --list-tools works without a project

Trailers

Per CLAUDE.md, the feature commit carries:

Implements: REQ-007
Refs: FEAT-010

The docs commit is Trace: skip (docs-only, exempt per the AGENTS.md
trailer policy).

🤖 Generated with Claude Code

[github-actions]

📐 Rivet artifact delta

No artifact changes in this PR. Code-only changes (renderer, CLI wiring, tests) don't touch the artifact graph.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Rivet Criterion Benchmarks'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 681563c	Previous: 92ad95d	Ratio
link_graph_build/10000	36870006 ns/iter (± 4159555)	30052745 ns/iter (± 2252438)	1.23
query/10000	141563 ns/iter (± 4674)	109024 ns/iter (± 1298)	1.30

This comment was automatically generated by workflow using github-action-benchmark.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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