Support `select_keys` in regex categorization and tagging rules

[TimeToBuildBob]

Problem

The Python query engine already supports restricting regex classification to specific event fields via select_keys (aw-core/aw_transform/classify.py).

aw-server-rust does not. The query parser currently reads regex and ignore_case, but ignores select_keys, and the matcher checks all string-valued fields on the event.

That means a rule intended to mean "match only app" or "match only title" silently becomes "match any string field" on the Rust stack.

Current Code Paths

• aw-query/src/datatype.rs parses regex rules from query JSON, but only handles regex + ignore_case
• aw-transform/src/classify.rs matches regex rules against every string value in event.data

Requested Behavior

• Parse optional select_keys: string[] from regex rules
• When select_keys is present, only test those event fields
• When select_keys is absent, preserve the current all-string-fields behavior
• Apply the same semantics to both categorization and tagging

Why This Matters

aw-webui can expose field selection in category rules, but that UI would be misleading until the Rust backend has parity with Python behavior.

Related threads:

• Ability to match category by Editor project? aw-webui#619
• Support editor-bucket-aware categorization for editor project/workspace rules activitywatch#1305

Test Coverage

Please add tests for:

• valid select_keys
• invalid select_keys types
• missing keys / non-string values
• legacy rules without select_keys

[TimeToBuildBob]

Opened PR #605 with the backend parity fix for select_keys, including parser validation and matcher coverage. Targeted tests: cargo test -p aw-transform -p aw-query.