[Bug] Make isolated Cron sessions read-only

[mikemikimike]

Scope

Scope boundary: Prevent isolated Cron sessions from accepting follow-up turns through the Gateway or Web UI.

Non-goals: Change Cron scheduling, transcript display, or interactivity for chat, CLI, channel, or subagent sessions.

Branch

Base branch: main

Target exception: N/A

Issue

Linked issue: Fixes #1520

Release Note

Release note: NONE

Tests

Ruff: uv run --frozen ruff check src/opensquilla/gateway/rpc_sessions.py tests/test_gateway/test_rpc_sessions.py passed. The pinned environment's Ruff formatter reports the two existing files would be reformatted by its newer formatter; unrelated full-file formatting was not included.

Pytest: uv run --frozen pytest tests/test_gateway/test_rpc_sessions.py -q --tb=short passed (301 tests). The full PYTHONUTF8=1 uv run --frozen pytest -x -vv --tb=long run reached 1,672 passed and 67 skipped before the pre-existing tests/test_ci/test_rpc_architecture_contracts.py::test_f2_foundation_runtime_stays_within_explicit_ceiling failure (1,216 lines versus the 1,200-line baseline ceiling); the same failure reproduces on origin/main.

Build: npm run build and uv build --wheel passed.

Regression tests: added

Notes: The default test path remains offline, deterministic, credential-free, and safe for forks.

Maintainer Live Check

Maintainer live check: no

Surface: N/A

Maintainer-only note: contributors are not expected to provide secrets or run credentialed live checks. Maintainers may run Live Release E2E for provider, browser, gateway, channel, or release smoke coverage.

Safety

No secrets, local-only artifacts, private prompts/transcripts, channel identifiers, or AI session artifacts are included.

Third-Party Origin

Third-party origin: none

Documentation Changes

• Links point to existing repository files or stable external pages.
• Code fences and Markdown tables render correctly on GitHub.
• Examples avoid real secrets, local private paths, and private transcripts.