fix(runtime-host): add numbered suffixes to branch session titles

[sunrioa]

Summary

Fixes #4978

Ordinary session branches currently inherit the source title, making them difficult to distinguish in the session list.

This change:

• Assigns the first available numbered title, such as Review project (1) and Review project (2).
• Avoids stacking generated suffixes when branching from an existing branch.
• Preserves manually assigned titles, including numeric endings such as Sprint (2026).
• Accounts for existing session titles, including archived sessions.
• Reserves names across concurrent branch creation requests and respects the session title's Unicode code-point limit.
• Preserves existing naming behavior for source sessions, revisions, and side conversations.

Before

After

Verification

Passed:

• Build, lint, formatting, typecheck, and Desktop/UI unused-code checks.
• All 552 script tests.
• All 33 Electron end-to-end tests.
• CLI packaging and offline installation smoke tests.
• Production dependency audit: no vulnerabilities reported.
• Relevant architecture, release-contract, license-header, locale, and protocol checks.
• Manual testing of the branch naming behavior by the author.

Known validation limitations

The complete local validation was not entirely green. Intermittent Host process-exit failures occurred in:

• an invocation-owned detached Host retires after launcher natural exit
• an authority-supervised Candidate exits if its launch owner is killed
• owned Host exits promptly after its first connection closes

Issue #4776 tracks the latter two tests, with test-side lifecycle changes proposed in #4814. Additional investigation captured a pending TLS connection from the startup model-metadata refresh remaining alive after host.closed resolved; see the diagnostic comment. This explains the instrumented authority-supervised failure, but has not been confirmed as the cause of every reported failure. The first test also failed on unmodified main and is not explicitly listed in #4776.

Three independent worktrees were created with fresh dependency installations and rebuilt artifacts:

Version	Full Host suite
Original base: 2dd98419a	1781 passed, 12 skipped
Original base with this patch	1782 passed, 1 failed, 12 skipped
Unmodified main: f4abd4918	1790 passed, 12 skipped

Five additional rounds of the three lifecycle tests reproduced all three failure types on unmodified main.

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex assisted with implementation, regression tests, ablation experiments, and validation. The author manually tested the user-visible behavior.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Static checks and branch-specific tests pass. The complete Host suite encountered the intermittent failures documented above.

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[me2seeks]

Automated review by OpenAI Codex, operated by me2seeks, at 20f83123b2e5a53f37bb76abaf57e7c7aad741f3. This is not an independent human review.

One P2 compatibility finding: the suffix-stripping heuristic cannot distinguish a new generated branch suffix from an inherited title in an existing branch. See the inline exact-source reproduction.

1. Optimal for the actual problem: mostly; naming is correctly owned by the Host and cross-source branch reservations are serialized, but existing branch title provenance is misclassified.
2. Production code that can be deleted: no closed deletion candidate. Do not repurpose titleIsManual as proof that a numbered suffix was generated by this feature.
3. Low-quality tests that can be deleted or replaced: none identified. Current numeric-title coverage uses manual names and misses persisted branches with auto-generated literal numeric endings.
4. Deeper refactor: no broad refactor; remove a suffix only when its generated provenance is established, preserving ambiguous existing names.
5. Ready to merge: no, pending correction of the existing-data naming case.
6. Residual risks / verification: read all three changed files, branch/revision/side-conversation construction, idempotent retries and stored title-update semantics; executed the exact nextBranchName function on a valid legacy header shape. Full Host branch integration suites were not run locally. User-visible title behavior and persisted metadata are affected; material protected-area changes require independent human review under CONTRIBUTING.md.