docs(product): remove shipped items from roadmap

[TheGreatAxios]

Summary

• Remove the Roadmap section from docs/PRODUCT.md: both entries describe shipped behavior, so a planned-not-yet-shipped section no longer has content
• Fast provider/model switching with persisted default is covered by the /model surface (Alt+D persist, per-repo selection file, immediate cutover)
• Perpetual-session context management is covered by the compactor plus continuation in TUI and exec

Verification

• bun run lint, bun run typecheck, and bun run build pass
• Full bun run check test phase shows 7 fail plus 1 error, all in files untouched by this change; every affected file passes in isolation (pre-existing order-dependent flakes, unrelated uncommitted work sits in the shared checkout)
• git diff origin/main...HEAD is docs-only (5 deletions in docs/PRODUCT.md)

Fixes CL-7933

[linear-code]

CL-7933

[TheGreatAxios]

Critic · Approve

Removes the shipped-only Roadmap section from docs/PRODUCT.md; no code changes.

Findings

• docs/PRODUCT.md:112,116 — both removed bullets duplicate shipped behavior documented in the same file: Alt+D persists the default, switch cuts over immediately, per-repo selection file. Backed by src/session/live-model-switch.ts (applyLiveModelSwitch) and src/tui/runner-host.test.ts:429 (Alt+D sets default).
• src/session/compactor.ts:1-13 — deterministic compactor curating inference-facing context while history stays complete; continuation wired in src/session/runtime-assembly.ts:421. Perpetual-session management is shipped, not planned.
• gh pr diff 1020 is docs-only (docs/PRODUCT.md, section header + 2 bullets removed). No dangling Roadmap references elsewhere; surrounding sections flow cleanly.

Notes

• No blocking issues. Merge-ready. CI test-phase failures cited in the PR body are unrelated to a docs-only diff; no re-run needed beyond the author lint/typecheck/build.