feat: time awareness for agent #63

[ypldan]

Applicable issues

• fixes Time awareness for agent #63

Description of changes

Gives the agent a sense of time through four cooperating mechanisms:

1. Auto-injection — every user turn gets a synthetic current_timestamp tool-call/result appended to the message list, so the agent always knows "now" without an explicit tool call. Historical timestamps survive the state round-trip with their original times, giving the agent temporal progression across turns.
2. Explicit timezone tool — the agent can call current_timestamp with an IANA timezone parameter (e.g. Asia/Tokyo) for authoritative server-side conversion, avoiding LLM arithmetic on DST and offset rules.
3. Tool response metadata — every tool result is stamped with its production time after execution. A per-invocation annotation transformer appends a human-readable [Timestamp: ...] line to tool messages before each LLM call, so the agent can reason about data freshness. Annotations are transient — they never leak into the persisted message history.
4. Two-tier transformer pipeline — introduces PreInvocationTransformer (runs before every LLM call, selective deep-copies) alongside the existing MessagesTransformer (runs once at setup, mutates canonical history). _AttachmentFilter is refactored into this new tier. A CompletionResultEnricher ABC is added for post-execution enrichment in ToolExecutor.

The entire feature is preview-gated (@preview_module + PreviewField) under ENABLE_PREVIEW_FEATURES. The features.timestamp config field defaults to enabled when preview is active, and can be explicitly set to null to disable per-app.

Checklist

• Title of the pull request follows Conventional Commits specification
• Design documented is updated/created and approved by the team (if applicable)
• Documentation is updated/created (if applicable)
• Changes are tested on review environment
• App schema changes are backward compatible, or breaking changes are documented with a migration guide
• Integration tests pass

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ypldan]

/deploy-review

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