fix(history): preserve long opening task contracts

[ousamabenyounes]

Protected opening messages keep the static-slab cache anchor, but their task text was reduced to a 300-character preview. Long enumerated contracts therefore lost later requirements even though the opening message itself remained native.

Tasks above the existing native-text budget now use the existing bounded task representation instead of the compact preview. The prior-context tombstone remains in place, so this does not turn the opening task back into the live request.

Fix #233

Test verification (RED → GREEN)

Upstream main with the new regression test:

FAIL  tests/history.test.ts > collapseHistory — opening-turn request quarantine (regression #14) > preserves every requirement in a protected enumerated opening task
AssertionError: expected ... to contain '4. MUST implement requiredApiFunction3; map InvalidArgument to TypeError; preserve boolean SELECT/expression invariants.'
Test Files  1 failed (1)
Tests  1 failed | 56 skipped (57)

Patched branch:

Test Files  1 passed (1)
Tests  1 passed | 56 skipped (57)

Full validation:

pnpm install --frozen-lockfile
pnpm run audit
No known vulnerabilities found
pnpm run typecheck
pnpm test
Test Files  75 passed (75)
Tests  1204 passed (1204)
pnpm run build
✓ emitted dist/ library modules + declarations
✓ built dist/node.js
✓ version smoke check: --version prints 0.13.2

• Rebased on current main
• pnpm test and pnpm typecheck pass
• No raw prompts, credentials, session files, or machine identifiers