Trim dead exports and consolidate duplicated helpers - #85

Salvages the high-signal portion of the parked
`cleanup/tier1-dead-exports` branch (commit `e2285b3`, May 12),
re-derived on top of post-rename `main`. No "cc-shell" strings
introduced.

Two threads of cleanup land together.

## Dead exports / dead returns

- src/main/window/mainWindow.ts — delete `getMainWindow()` (zero
  importers internal or external).
- src/main/sessionManager.ts — privatize `ManagerEvents`,
  `SpawnOptions`, `SpawnResult`. These three types are consumed only
  by the EventEmitter overloads on the SessionManager interface in
  the same file. WHY-comments capture why they should stay private.
- src/renderer/.../claudeImages.ts — privatize 5 internal-only
  symbols: `MAX_CLAUDE_IMAGE_BYTES`, `SUPPORTED_CLAUDE_IMAGE_MEDIA_TYPES`,
  `fileToDataUrl`, `parseDataUrl`, `estimateBase64DecodedBytes`. The
  only consumer (useClaudeImagePaste) only pulls the predicates and
  ingress functions. WHY-comment documents the public surface.
- src/providers/claude/runtime/claudeSession.ts — remove the
  `terminalToMarkdown` re-export. It was a compat shim from when the
  function lived inline; today its sole definition is in
  src/shared/runtime/ptyScreen.ts and nothing imports it via
  claudeSession. HISTORICAL note left in place.
- src/renderer/.../useClaudeImagePaste.ts — drop `appendDraftImages`
  from the hook's return object. The only caller (TileLeaf) only
  destructures `handlePaste` and `removeDraftImage`; the append
  helper is purely internal now (paste-only ingress).
- src/providers/codex/types/codexTranscript.ts — delete unused
  `CodexSessionMeta`, `CodexResponseItem`, `CodexEventMsg` types and
  the `isCodexConversationEntry` guard. Zero external imports;
  consumers handle `payload: unknown` with inline narrowing.

## New shared helpers + dedup call sites

Five new modules replace hand-duplicated copies:

- src/renderer/src/lib/relativeTime.ts — replaces 4 copies in
  PathPickerModal, PromptSearchModal, AgentActivityModal, and
  WorktreesBar. Picked the most complete variant (years + 'just now'
  branch). UX delta for callers that lacked 'just now': fresh
  timestamps now show 'just now' instead of '2s ago' — softer, no
  regression risk.
- src/renderer/.../features/workspace/lib/sessionDisplay.ts —
  consolidates `cwdBasename` + `providerGlyph` from AgentActivityModal
  and PromptSearchModal. Wider provider signature (claude|codex|terminal)
  so the agent-activity surface doesn't have to special-case.
- src/renderer/.../features/system-perf/formatBytes.ts — consolidates
  the two badge/popover copies of the SI byte formatter. Tunable
  `fractionDigits = 2` default.
- src/renderer/.../workspace/codex/eventCursor.ts —
  `codexTurnIdFromEventPayload` + `codexEventType`, replacing three
  copies across history.ts / initialHistory.ts / useIpcSubscriptions.ts.
  WHY-comment captures why the payload-level turn_id is load-bearing
  fallback evidence (pagination + IPC bursts can start after the
  defining `turn_context`).
- src/shared/lib/asRecord.ts — shared unknown-JSON narrowing helper.
  Replaces two copies in workspace/codex/rollout.ts and
  providers/codex/renderer/rows/CodexRows.tsx. NOTE: CodexRows' local
  copy did NOT exclude arrays — replacing with the stricter shared
  helper means arrays now return null. Every call site uses `.raw` /
  `.input` property reads that would be undefined on arrays anyway,
  so this is a free safety improvement, documented in a WHY-comment
  at the import.

## Verification

- npm run test:review-fixes — settings/prompt-templates/session-ownership all green
- npm run test:work-context — green
- npm run build:app — green
- git diff main | grep -ic "cc-shell" — 0

17 modified + 5 new files, +88/-263.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>