refactor(session): centralize fallback title policy

[kitlangton]

What

Centralize session-title fallback policy in the browser-safe @opencode-ai/util package so App, ACP, Core, Enterprise, and TUI no longer maintain subtly different constructors and recognizers.

This keeps four existing behaviors explicit and separate:

• timestamped compatibility titles for consumers that require a string
• compact labels for human-facing App surfaces
• broad recognition of absent or historical timestamped fallback titles
• exact root-session fallback recognition for title-generation retries

How

• Add session-title-fallback.ts with focused tests covering root, child, empty, historical, generated, and exact-match cases.
• Replace duplicated timestamp constructors in App normalization, ACP, Enterprise sharing, and TUI surfaces.
• Move App display-label normalization and TUI fallback recognition to the shared policy.
• Keep Core retry eligibility exact to the root session's creation timestamp.
• Remove the ambiguous App and TUI sessionTitle helpers.

flowchart LR
  S[Session info] --> P{Consumer policy}
  P -->|String required| T[withTimestampedFallback]
  P -->|Human label| D[displayLabel]
  P -->|Terminal or splash| H[isFallbackTitle]
  P -->|Core title retry| E[isExactRootFallback]

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Scope

• No protocol, schema, or generated-client changes.
• No persisted-data migration; historical fallback titles remain recognized.
• No visible title behavior changes.

Testing

• packages/util: bun run test, bun typecheck, bun run build
• packages/app: bun typecheck, focused src/utils/session.test.ts
• packages/cli: bun typecheck
• packages/core: bun typecheck, focused test/session-title.test.ts
• packages/enterprise: bun typecheck
• packages/tui: bun typecheck, focused test/util/session.test.ts
• Targeted Oxlint: 0 errors
• App display formatter benchmark, 10,000,000 calls: 184.14 ms before, 190.95 ms after, identical consumed output