Fix Codex subagent filtering and inspector entries

[calvintvu]

Context

Codex transcripts can include internal permission-review child sessions and newer nested subagent activity events. The collector exposed internal checks as delegated work while omitting some genuine subagents from the session inspector.

Changes

• Exclude internal permission-review sessions using structured source metadata.
• Record nested subagent start events so genuine subagents appear consistently in the rail and inspector.
• Preserve root sessions and genuine thread-spawned subagents.

Test

• go test internal/vendors/codex/discovery.go internal/vendors/codex/fork.go internal/vendors/codex/metadata.go internal/vendors/codex/parse.go internal/vendors/codex/types.go internal/vendors/codex/parse_test.go -count=1 — passed
• go build ./... — passed
• git diff --check — passed

Screenshots

• Session views — genuine subagent appears in both the rail and inspector

• Permission review — internal check is absent from both views
  before

after