refactor: remove dead code and frontend cruft (improvement plan phase 1) (#611)

Phase 1 of the gitguardex improvement plan: dead-code removal only, no
behavior or public CLI/API change. 26 files, ~6274 deletions.

Deletes (not shipped to npm, zero runtime requires):
- frontend/Recodeeplan-handoff.tar.gz + frontend/scripts/** (stale forks of templates/scripts/)
- src/agents/detect.js (superseded by registry.getAgentDefinition)
- src/cockpit/{layout,keybindings}.js (dead tmux-era modules)
- the four matching test files

Dead-export/function removals:
- terminal/index.js: selectTerminalBackendForTarget, resolveTargetBackendName + 5 private helpers
- terminal/tmux.js targetId export; context.js clearProbeCache (no callers)
- pr.js unused fs/path/TOOL_NAME imports + _internal/TOOL_NAME re-exports
- cli/commands/pr.js dead no-op block; doctor/index.js rawRun alias
- cli/args.js collectForceManagedPaths/parseRepoTraversalArgs exports

Deferred: git/index.js rawRun (base-branch WIP staged on main),
cockpit/index.js spread narrowing (belongs with Phase 4 lazy-require).

Tests: 548 pass / 25 fail / 1 skip (pre-existing baseline, zero new failures).

Co-authored-by: NagyVikt <nagy.viktordp@gmail.com>

Author: NagyVikt