fix(tui): bound sub-agent failure notes to the alert dwell

[jkyberneees]

Problem

Field report: a transient sub-agent SAX timeout note never disappears.
Root cause: subagentTerminalNote pushed error/timeout notes with a
zero expiry — sticky by design until the turn finalized. A turn that
kept streaming after the sub-agent died kept the note on screen
indefinitely; a turn that never finalized client-side kept it forever.

Fix

Failure notes (error, timeout) now surface at alert tier
(alertTTL, 10s) — same contract as every other alert in the strip:
long enough to read, guaranteed to fade. User-initiated cancels stay
transient (3s), unchanged.

Nothing is lost by the fade — the failure is permanently recorded
where it belongs:

• the agent chip on the delegate step (⏱ timeout / ✗ error)
• the turn's swarm verdict (sub-agents: 5 ✓ · 1 ✗ — #4 timeout)

The now-dead clearStickyNotes (whose sole purpose was retiring these
sticky notes at finalize) is removed; the strip now has exactly one
notice lifetime rule: every note has an expiry.

Tests (RED-first)

New subagent_timeout_note_test.go: timeout note exists, is non-sticky,
the agent card still records timeout, and the note is gone after
alertTTL; all terminal notes bounded. Existing
TestStickyFailureAndVerdict updated deliberately to the new contract
(TestFailureNoteBoundedAndVerdict) — verdict assertions unchanged.

Verification

• go test -race -count=1 ./... — 7/7 packages ok
• go vet / go fmt clean (golangci-lint via CI)