Adds a persistent, per-project summary that SessionStart injects at the
top of every new Claude Code conversation, so each fresh session opens
with a clear picture of what the project IS — not just what was decided
last week. Pairs with a bump from 1 → 3 prior-session rollups so the
"last 2-3 sessions of work" context the user asked for actually
surfaces.

What's added

- memory.db v4: new `project_summary` table (one row per project) with
  pitch, tech_stack, recent_focus, source_file_count, generated_at_epoch.
  Migration is purely additive — old dbs upgrade in place on the next
  `connect()`. The migration path doesn't need sqlite-vec, so users
  without it still get v4.

- `context_engine.memory.project_summary` — extractive builder with no
  LLM dependency:
    * pitch     ← first substantive line of README.md (HTML/badge
                  stripped), fallback to pyproject.toml's
                  [project].description
    * tech_stack ← extension tally across the indexed chunks
                   ("Python (124), TypeScript (38), Markdown (12)")
    * recent_focus ← top 5 file_paths from the code_areas table,
                     most-recent first, with each description capped
                     at 100 chars
  Plus upsert / load / is_stale / format_summary_block helpers.

- `cce summarize` — manual refresh, `--force` to bypass the 7-day
  staleness check.

- `cce init` now calls `_refresh_project_summary` after the initial
  index so the summary lands before the first SessionStart fires.

- `build_session_resume` rewritten:
    * prepends the project summary block when present
    * shows the last 3 sessions' rollups (was 1) — header switches to
      "Previous N sessions" when plural, "Previous session" when one
    * tolerates an old db that's missing the v4 table (skips the block
      instead of raising)

- Tests (new file):
    * schema migration creates table on fresh and upgrades existing
    * upsert round-trips, replaces existing, returns None when absent
    * pitch extraction from README, fallback to pyproject, empty when
      neither present
    * tech_stack tally
    * recent_focus ordering
    * format_summary_block omits empty sections
    * build_session_resume:
        - includes the new summary block
        - shows the last 3 of 4 prior sessions and drops the oldest
        - uses singular "Previous session" with exactly 1
        - returns empty string on a virgin project
        - degrades gracefully when project_summary table is missing

Not in this PR (follow-up)

- Mirroring the same summary block into the MCP `context-engine-init`
  bootstrap prompt so Codex CLI (which doesn't have hooks) gets the
  same content via its system-prompt path. The data and the formatter
  are now ready for that wiring.

Suite: 894 passed, 1 skipped, 0 failed. Ruff clean.