feat(start): add --hide flag and focus filter subcommand for tool visibility

[samuelds]

Summary

• Adds --hide=<patterns> flag to focus start to hide specific tools from the AI client at launch (comma-separated, trailing-* glob supported)
• Reads ~/.focus/config.json tools.hidden array as persistent default; CLI flag overrides config
• Adds focus filter hide/show/list/clear CLI subcommand to manage the hidden list from the terminal
• Adds focus_filter MCP tool so agents can manage their own toolset visibility from within the AI client
• focus_filter is always visible regardless of the hidden list (prevents deadlock where agent can't re-show tools)

Motivation

Previously focus start exposed all tools or none (FOCUS_BENCH_MODE). Users wanting a curated toolset without uninstalling bricks had no option. This PR adds a blacklist model: everything is exposed by default, and users opt-in to hiding specific tools.

Examples

# Hide a single tool
focus start --hide=sym_get

# Hide an entire family
focus start --hide="focus_*"

# Persist the hidden list (applies on next focus start)
focus filter hide sym_get
focus filter list
focus filter clear

# From the AI client (via MCP)
focus_filter action=hide pattern=sym_get
focus_filter action=list

Test plan

• matchesPattern unit tests (exact + glob)
• isHiddenTool unit tests (null list, patterns, focus_filter immunity)
• --hide=sym_get hides sym_get from tools/list
• --hide=focus_* hides focus_* but keeps focus_filter visible
• No flag → all tools exposed (default behaviour)
• Dispatcher rejects calls to hidden tools
• Config file tools.hidden used when no CLI flag
• CLI --hide overrides config file
• No filter → all tools exposed
• All 240 tests pass, TypeScript strict, Biome clean

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