feat: add context input telemetry event helper

[caioribeiroclw-pixel]

What

Adds an opt-in TelemetryHooks.record_context_input_loaded(...) helper that emits a context.input.loaded event on the active session span.

The event is intentionally privacy-preserving: callers pass paths/URIs, hashes, and categorical metadata, not raw prompts, raw context bodies, tool arguments, memory contents, secrets, or full transcripts.

Why

claude_telemetry already makes tool/model execution observable. For headless Claude Code / CI runs, another useful debugging question is: which repo instructions, skills, hooks, or memory records entered the session before the model/tool spans happened?

This is a small step toward the idea from #8 without changing default behavior or requiring the wrapper to inspect project context.

Checks

• python3 -m py_compile claude_telemetry/hooks.py tests/test_hooks.py
• git diff --check

I could not run pytest/ruff in this container because Python packaging tools are not installed (pytest: command not found, python3 -m pip: No module named pip, and python3 -m venv is missing ensurepip).