feat(desktop): add state-aware Git next action

[IchenDEV]

Summary

• replace the fixed Git header command with one state-aware next action
• share the same resolved action and explanation between the session header and Git dock
• keep loading, unsupported, degraded forge, review, merge, and worktree-cleanup states honest and guarded
• preserve existing Source Control, Push, Pull Request, and confirmed cleanup handlers

Interaction behavior

The priority is local changes, unpushed commits, change-request creation, conflicts, failed checks,
requested changes, pending checks, remote updates, draft/review state, merge readiness, and merged
worktree cleanup. The chevron exposes only distinct valid alternatives. A merge-ready action is
labelled “Review & merge PR” because it opens the existing guarded PR surface rather than merging
immediately.

Verification

• bun test: 850 passed, 0 failed, 5,039 expectations across 147 files
• focused Git/header/dock tests: 18 passed, 93 expectations
• bun run build:renderer: ESLint, Stylelint, TypeScript, and 6,600-module Vite build passed
• light, dark, and 820x760 renderer acceptance passed with no browser warnings or errors
• docs, SDLC, worktree SDLC, Gate tests, and git diff --check passed

Conflict boundary

This branch is based directly on the current origin/main and is clean against it. PR #215 overlaps
this change in App.tsx and strings.ts. A read-only combined merge simulation reports an
App.tsx conflict while strings.ts auto-merges; PR #215 is already DIRTY against current main
because of additional TaskBoard conflicts. Resolve #215 onto current main before combining the two
changes.

Lifecycle

The implementation and rendered interaction were accepted by Chenli on 2026-09-01. Merge and
release remain separate Gates. See
docs/sdlc/changes/2026-09-01-git-next-action/change.md.