feat(mcp): --tools flag to select exposed MCP tools (#716)

[gustav-fff]

Closes #716

Root cause

fff-mcp unconditionally registers find_files, grep, and multi_grep in the rmcp ToolRouter and ships the full 75-line MCP_INSTRUCTIONS blob (crates/fff-mcp/src/main.rs:19-93 on pre-fix main). Every client pays the schema + instructions cost even if it only needs one tool.

Fix

Add --tools <name,...> (clap ValueEnum, snake_case, comma-separated, default = all three).

• FffServer::new now takes the exposed set, filters the router via ToolRouter::remove_route, and stores instructions built by a new build_instructions() helper.
• Unknown tool names cause clap to fail startup with the list of valid values.
• Instructions are assembled dynamically: sections that reference disabled tools (e.g. rule 4 about multi_grep, grep/multi_grep constraint syntax) are omitted so the model is not told to use tools it cannot call.

Behavior with no flag is identical to before.

Steps to reproduce

Before the fix, on origin/main:

cd fff
cargo run -p fff-mcp -- --help 2>&1 | grep -- --tools
# no output — flag does not exist

# Even a client that only wants find_files:
cargo run -p fff-mcp
# advertises find_files, grep, multi_grep + 75 lines of MCP_INSTRUCTIONS

After the fix:

cargo run -p fff-mcp -- --tools find_files --help
# lists --tools with valid values

cargo run -p fff-mcp -- --tools bogus
# error: invalid value 'bogus' for '--tools <TOOLS>...'
#   [possible values: find_files, grep, multi_grep]

cargo run -p fff-mcp -- --tools find_files,grep
# advertises only find_files + grep; instructions omit multi_grep rules

Expected: unknown names hard-fail at startup; enabled subset is the only thing list_tools returns; get_info().instructions no longer mentions the disabled tools.

How verified

$ cargo test -p fff-mcp
running 17 tests
...
test server::tests::router_defaults_to_all_three_tools ... ok
test server::tests::router_exposes_only_find_files_when_requested ... ok
test server::tests::router_exposes_only_grep_pair_when_multi_grep_dropped ... ok
test instructions::tests::default_all_tools_mentions_all_three ... ok
test instructions::tests::grep_without_multi_grep_drops_multi_grep_rule ... ok
test instructions::tests::multi_grep_only_keeps_multi_grep_rules ... ok
test instructions::tests::only_find_files_drops_grep_and_multi_grep_sections ... ok
test result: ok. 17 passed; 0 failed

cargo clippy -p fff-mcp --no-deps clean; cargo fmt applied.

Automated triage via Gustav. Honk-Honk 🪿

[dmtrKovalenko]

The implementation is pretty sloppy but it makes sense @gustav-fff can you please make at least comments less sloppy

[gustav-fff]

[triage-bot] DIRECTED: pushed 1dcdc2b.

• `ExposedTool` + `tool_name` now `pub(crate)`; replaced `fn all()` with a `const ALL`.
• Dropped the `pub use build_instructions` re-export; `server.rs` imports it directly.
• `FffServer::new` iterates `ExposedTool::ALL` instead of a hardcoded 3-item list.
• Trimmed the `--tools` clap doc from 2 lines to 1 (clap already prints the valid values on error).

`cargo test -p fff-mcp` 17/17, `cargo clippy -p fff-mcp --no-deps` clean.

Honk-Honk 🪿