feat(context-rewrite): add shared observability events

[lingsanbuer]

Summary

• add six shared context rewrite lifecycle events
• persist sanitized events through the existing event-trace.jsonl store
• enforce a whitelist-only event boundary for persisted observability data
• summarize content using SHA-256 digests and character counts
• redact unsafe free-text reason and error values into digest codes
• add tests for event definitions, normalization, redaction, and trace persistence

Events

• context_rewrite_planned
• context_rewrite_validated
• context_rewrite_applied
• context_rewrite_deferred
• context_rewrite_failed
• context_rewrite_bypassed

Privacy

Raw payloads, headers, authorization values, source text, and error messages are not persisted. Content samples are
converted to SHA-256 digests with character counts before writing.

Scope

This PR only changes the shared host-adapter observability boundary. It does not modify the Claude Code, Codex, or
OpenClaw adapters.

Validation

• targeted observability tests: 6 passed
• host-adapter tests: 48 passed
• host-adapter typecheck
• package boundary validation
• workspace recursive typecheck
• git diff --check