Make MCP metadata configurable via --tools/--prompts/--examples overrides

[millerforce-bot]

What this PR does

This adds optional runtime overrides for the MCP server metadata that is currently static in server.json and compiled into the binary.

New clippy mcp-server flags:

• --tools <path>: override MCP tool descriptions + argument descriptions
• --prompts <path>: override MCP prompt descriptions + argument descriptions
• --examples <path>: override MCP examples shown to clients

Why this is useful

• Enables teams to adapt Clippy's MCP UX to local conventions and workflows without forking behavior code
• Keeps tool handlers stable while allowing prompt/tool wording to evolve quickly
• Makes it easier to tune assistant guidance for different clients/environments

Validation behavior

• All override files are validated at startup
• Invalid JSON fails fast with a clear error
• --tools validation requires complete coverage of every MCP tool + every expected argument description
• --prompts validation requires complete coverage of every MCP prompt + every expected argument description
• Unknown/missing tools, prompts, or arguments fail startup

Included artifacts

• New metadata loader and override validator in cmd/clippy/mcp/metadata.go
• MCP server now consumes metadata dynamically in cmd/clippy/mcp/server.go
• New CLI flags for mcp-server in cmd/clippy/main.go
• Example override files:
  • examples/clippy.mcp.tools.json
  • examples/clippy.mcp.prompts.json
  • examples/clippy.mcp.examples.json
• server.json updated to include buffer_cut so metadata stays in sync with server tools

Usage

clippy mcp-server \
  --tools examples/clippy.mcp.tools.json \
  --prompts examples/clippy.mcp.prompts.json \
  --examples examples/clippy.mcp.examples.json

Notes

• No behavior changes to the underlying tool handlers.
• This is metadata/configuration flexibility only.

[neilberkman]

@mmmnorthmark thanks for the contribution. Will be released in 1.6.5.