feat: review inbox with state-tracked PR comments

[eugenioenko]

Summary

• Adds a Review Inbox sidebar panel (Ctrl+K V) that treats PR comments as tracked tasks with states: Open -> Addressed -> Verified -> Dismissed
• Auto-detects "addressed" comments by checking git log for file modifications after the comment timestamp
• Shows gutter markers (diamond indicators) on commented lines in the editor, with colors reflecting comment state
• Navigation commands (Ctrl+K N / command palette) let you jump through unresolved comments across files like diagnostic jumping
• Status bar shows live progress (e.g. "3/7 resolved") when a review is active
• State persists locally in .ttt-review-state.json so review progress survives across editor sessions
• Comments auto-load when a PR is opened via the Changes panel

Architecture

• internal/github/github.go -- new FetchPRComments, AddPRComment, AddPRInlineComment functions using gh CLI
• internal/github/review_state.go -- CommentState enum, ReviewState persistence, DetectAddressed heuristic
• internal/ui/review_inbox_widget.go -- sidebar panel widget with file-grouped comments, state indicators, and keyboard shortcuts (v=verify, d=dismiss, r=reopen, a=reply)
• internal/app/review.go -- app-layer wiring: async comment fetch, state management, gutter marker updates, navigation
• internal/app/commands_review.go -- command palette entries for all review actions

Design Decisions

• Comments are tasks, not messages: each thread has a trackable state
• The editor shows current code, not stale diffs -- gutter markers highlight where comments exist
• Navigation wraps around like diagnostic jumping (next/prev cycle through all files)
• State detection is best-effort (checks if file was modified after comment) with manual override
• Uses existing patterns: SelectableList, BaseWidget, PostEvent for async, term.Style* constants for colors

Test plan

• Unit tests for GitHub API JSON parsing (internal/github/github_test.go)
• Unit tests for review state management (internal/github/review_state_test.go)
• Unit tests for inbox widget: comments, state transitions, progress, markers, expansion (internal/ui/review_inbox_widget_test.go)
• E2E sidebar tab overflow test updated for new panel
• make build && make test passes
• Manual: open a PR URL, verify inbox populates and gutter markers appear
• Manual: use v/d/r keys to change comment states
• Manual: use Ctrl+K N to navigate through unresolved comments

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