feat: harden telemetry privacy filters and anon ID policy

[abrichr]

Summary

• harden privacy filtering by scrubbing top-level message and logentry fields
• scrub context/tag values and enforce a strict observability tag allowlist
• switch anonymization to versioned HMAC IDs (anon:v2:*) with per-install salt creation/persistence
• make git-based internal classification opt-in via OPENADAPT_INTERNAL_FROM_GIT
• enforce final privacy scrubber ordering for custom before_send hooks
• update docs and add regression tests for all new behavior

Validation

• uv run ruff check src/openadapt_telemetry/client.py src/openadapt_telemetry/config.py src/openadapt_telemetry/privacy.py tests/test_client.py tests/test_config.py tests/test_privacy.py
• PYTHONPATH=src uv run pytest -q

Notes

• This PR is stacked on top of feat: make telemetry opt-out and enforce anonymized user IDs #3 (fix/add-readme-badges) to avoid mixing with unrelated in-flight work.