feat: Compress MCP tool output via PostToolUse hook

[jmeiracorbal]

About

Adds a PostToolUse hook that compresses MCP tool responses before they reach Claude's context. Unlike Bash commands (intercepted via PreToolUse), MCP tool outputs were previously untouched by rtk.

Claude Code supports updatedMCPToolOutput in PostToolUse hooks which allows replacing a tool's response before the agent sees it. This is the only deterministic interception point available for MCP tools (it doesn't depend on agent cooperation like Read actions).

How it works

1. rtk init --global installs rtk-post-tool-use.sh alongside the existing rtk-rewrite.sh and registers it under PostToolUse in ~/.claude/settings.json with matcher mcp__.*.
2. On every MCP tool call, the hook pipes the PostToolUse JSON to rtk filter-mcp-output.
3. The subcommand deduplicates repeated lines and truncates responses over 3000 chars, returning updatedMCPToolOutput only when there is actual savings — pass-through when the response is already short.
4. Errors are logged to ~/.local/share/rtk/logs/mcp-filter.log and the hook always exits 0 so it never blocks Claude.

Changes

• src/mcp_filter.rs: Compression logic (deduplicate + truncate).
• hooks/rtk-post-tool-use.sh: The same behavior that ~/.claude/hooks/rtk-rewrite.sh.
• src/main.rs: filter-mcp-output subcommand.
• src/init.rs: Can install/uninstall PostToolUse hook alongside existing PreToolUse hook.

Test plan

• cargo test: 615 passed, 0 failed.
• Manually verified hook fires for mcp__.* tools in Claude Code.
• Verified compression triggers on responses > 3000 chars.
• Verified no output (pass-through) when response is under threshold.
• rtk init -g --auto-patch idempotent: Can detects hook already present.
• Errors logged to file, hook never blocks Claude on failure.

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