fix: guard repeated before-turn attempts per session

[compoodment]

Summary

• Replaces the dead global BeforeTurnKernel turn guard with a bounded per-session attempt signature map.
• Marks a BeforeTurnKernel attempt before the RPC starts, so repeated assemble calls after timeout/failure do not retry the same session turn immediately.
• Builds the signature from the original normalized transcript shape and explicit message IDs only, avoiding synthetic UUID churn from normalized messages.
• Adds regression coverage for same-session duplicate failed attempts and same-text turns in different sessions.

Fixes #312.

Verification

• pnpm exec tsc --noEmit — PASS
• pnpm run clean:test && pnpm exec tsc -p tsconfig.tests.json && node --test --test-name-pattern="beforeTurn attempt" .ts-build/test/unit/context-engine.test.js — PASS
• pnpm check — FAILS after TypeScript/plugin inspector, in existing unrelated context-engine/memory-provider/slot-conflict unit tests; the new beforeTurn tests pass in that run.
• npm run test:integration — FAILS in existing unrelated checklist/host-flow expectation tests.

Stack note: this branch includes #318 so test compilation is unblocked before this focused before-turn change.

– Vale

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

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Run ID: 47a72d23-f287-4ed3-af20-0eaae2d80bbc

📥 Commits

Reviewing files that changed from the base of the PR and between 6d47279 and 8708846.

📒 Files selected for processing (3)

• src/context-engine.ts
• test/integration/markdown-ingest.test.ts
• test/unit/context-engine.test.ts

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