Add inline pane rename from the sidebar

[zordhalo]

Description

Panes can now be renamed inline from the sidebar: double-click a pane's row, type a new name, press Enter. Escape cancels, an empty or unchanged name is discarded rather than saved, and clicking outside the field commits the edit.

The backend for this already existed — sessions:rename (main/src/ipc/session.ts), API.sessions.rename, and a full set of handleStartEditName / handleSaveEditName / handleCancelEditName handlers in useSessionView.ts that were exported but never rendered by any component. This PR is the missing UI, plus one store fix it surfaced.

Two implementation notes worth a reviewer's attention:

1. Commit is driven by pointer, not blur. PanelTabStrip commits its tab rename on onBlur, but that pattern breaks here: double-clicking a row also activates the pane, and activation pulls focus to the terminal. The input received a blur one tick after mounting and committed itself instantly. Instead, a capture-phase pointerdown listener commits on a real outside click, and the first (programmatic) blur reclaims focus once.

2. sessionStore.updateSession had a pre-existing bug. A main repo session is stored twice — in the sessions array the sidebar renders, and in activeMainRepoSession. updateSession returned early after updating the active copy, so the sessions copy went stale. This affected any session:updated for the active main-repo pane (status, favorite, git metadata), not just renames; renaming simply made it visible. Both copies now update together, matching what updateSessionGitStatus in the same file already did.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation update

Checklist

• I have read the CONTRIBUTING.md guidelines
• My code follows the code style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have run pnpm typecheck and pnpm lint locally
• I have tested the Electron app locally

Critical Areas Modified

• State management/IPC events — sessionStore.updateSession (see note 2 above)

Testing

New tests/sidebar-rename-pane.spec.ts covers four cases: Enter commits, Escape cancels, an empty name is discarded, and renaming the active main repo pane updates the sidebar label. tests/electronApiMock.ts gains a sessions.rename handler that records calls and emits session:updated.

The main-repo test is the regression guard for note 2 — I stashed the store fix and confirmed that test fails without it and passes with it, so it is load-bearing rather than decorative. Note that the other three tests pass either way: they exercise a non-main-repo pane, which takes the other branch. The bug lived entirely in the branch no test covered.

• pnpm exec playwright test tests/sidebar-rename-pane.spec.ts tests/sidebar-compact.spec.ts tests/accessibility.spec.ts tests/smoke.spec.ts — 38 passed
• pnpm lint — clean
• pnpm typecheck — clean
• Manually verified in the Electron dev app against an isolated PANE_DIR

Documentation

• CHANGELOG.md — Added + Fixed entries under Unreleased
• README.md — new Usage step for renaming a pane
• docs/STATE_MANAGEMENT.md — new "Main Repo Sessions Are Stored Twice" section, so the dual-storage trap is written down rather than rediscovered

https://claude.ai/code/session_019yor34qu8TA9jTkkHtvu59