feat(agent-core): add session handover context and context-usage self-awareness

[WinterYukky]

Summary

Add two pure, side-effect-free helpers that support model-driven session
handover: work-state/handover-context extraction, and a per-turn context-window
usage self-awareness block.

Motivation

When a session hands over to a successor, the successor needs a concise seed of
the prior work (branches, PRs, repos, commits, recent conversation) so it can
continue without manual archaeology. Separately, surfacing the current
context-window utilisation back to the model lets it decide to hand over
before the window fills, rather than hitting truncation/overflow.

Changes

• packages/agent-core/src/lib/handover-context.ts: extract work state from recent
  conversation/tool activity and build the successor seed message + title.
• packages/agent-core/src/lib/auto-handover.ts: build the per-turn context-usage
  environment block (soft ~80% guideline; the model decides, no forced auto-fire).
• matching handover-context.test.ts / auto-handover.test.ts.

Testing

• agent-core unit tests: handover-context 26 + auto-handover 6 = 32 tests, all
  passing (tsc + prettier + vitest).

Notes

• Self-contained, pure (no AWS SDK), unit-testable in isolation. Independent of
  the other Batch 2 PRs (no shared files).

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