feat(tui): Phase 2 detail screens — Issue Detail, MR Detail, discussion tree, cross-refs
Implements the remaining Phase 2 Core Screens:
- Discussion tree widget (view/common/discussion_tree.rs): DiscussionNode/NoteNode types,
expand/collapse state, visual row flattening, format_relative_time with Clock trait
- Cross-reference widget (view/common/cross_ref.rs): CrossRefKind enum, navigable refs,
badge rendering ([MR]/[REL]/[REF])
- Issue Detail (state + action + view): progressive hydration (metadata Phase 1,
discussions Phase 2), section cycling, description scroll, sanitized GitLab content
- MR Detail (state + action + view): tab bar (Overview/Files/Discussions), file changes
with change type indicators, branch info, draft/merge status, diff note support
- Message + update wiring: IssueDetailLoaded, MrDetailLoaded, DiscussionsLoaded handlers
with TaskSupervisor stale-result guards
Closes bd-1d6z, bd-8ab7, bd-3t1b, bd-1cl9 (Phase 2 epic).
389 tests passing, clippy clean, fmt clean.
050e00345a