refactor(core): split mutation, MCP, TUI, and desktop-bridge modules

[IgorArkhipov]

Summary

• Split mutation.rs provider handlers into mutation/providers/* plus shared directory, path-file, JSON MCP, and toggle-dispatch modules. Backup, restore, and lock stay in the parent.
• Split MCP tool handlers into mcp/{inventory,toggle,control,groups,agent_plugins,workflows,schema,selectors,doctor}.rs. handle_tool_call stays in the parent.
• Continue the TUI and desktop-bridge splits: inventory and restore leave tui.rs; workflow, groups, agent plugins, restore, and snapshot leave desktop_bridge.rs.
• Cache desktop-bridge discovery for read-only snapshot/inspect paths. Plan, apply, and workflow validate/compile always refresh. Group, definition, and restore applies invalidate the cache. Plan snapshots are never reused for apply.

This PR is independent of #57, #58, and #59 and targets main. It overlaps #59 (toggle dispatch) and likely #58 (mutation.rs ID prefixes), so those should land first or this will need a rebase.

Skip lists (build/out/test/tests) are unchanged. Expanding or shrinking them is a product decision, not part of this refactor.

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --all-features --locked
• cargo run -p unpin-cli --locked -- --help
• Provider matrix / live-host validation not run (no provider behavior change)

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