fix(agent-observability): degrade gracefully when opentelemetry is missing

[alberthaolihai]

Summary

• core/agentlens/runtime.py imported opentelemetry at module scope with no guard, so any environment without the (optional) opentelemetry package crashed every agent-observability hook phase (SessionStart/UserPromptSubmit/PreToolUse/PostToolUse/Stop) at import time — before the existing try/except error handling in main()/runtime.main() ever got a chance to run.
• Fresh installs hit this reliably: run_hook.sh falls back to system python3 whenever the project .venv lacks zhiyanllm, and system python3 typically doesn't have opentelemetry installed — matching the traceback reported in [Bug]: ⚠ Hook SessionStart [warning] #17.
• Wrapped the import in try/except, consistent with how the rest of agentlens already treats opentelemetry/zhiyanllm as optional (bootstrap.load_runtime, tracing.generate_turn_carrier). The two call sites for _otel_trace are only reached once bootstrap.ensure_initialized has confirmed the runtime is available, so this is a no-op on the happy path.
• Regenerated the .cursor/.claude Classic host bundles via scripts/build-classic-hosts.py --write so all three editions stay in sync with the .codebuddy source.

This supersedes #28, which was opened from a stale branch that also carried unrelated, already-superseded commits and conflicted with main. This branch is based directly on current main and only touches the 3 files above.

Fixes #17

Test plan

• Reproduced the exact issue [Bug]: ⚠ Hook SessionStart [warning] #17 traceback against the pre-fix code using a clean venv with no opentelemetry installed.
• Confirmed the fix: same clean venv, all 5 hook phases (session-start/user-prompt-submit/pre/post/stop) now exit 0 with no traceback.
• pytest tests/ — 123 passed; the 6 pre-existing failures (test_cls_sink.py, test_collector.py) reproduce identically on the pre-fix code, confirming they're unrelated environment-dependent baseline failures, not regressions.
• python3 scripts/build-classic-hosts.py --check — Classic bundles match generator.
• bash scripts/validate.sh — passed (6/6).
• bash scripts/smoke-install.sh — passed.
• bash scripts/scan-secrets.sh — blocked; the bundled helper only supports Linux x86_64 and this was run on macOS.

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