fix(tui): route mid-session diagnostics through the TUI, never raw stderr

[Emasoft]

Any mid-session fmt.Fprintf(os.Stderr) write was painting raw text over bubbletea's alt-screen. Plugin hook warnings like [plugin/onToolCall/...] hook timed out and bash-gate hints garbled and duplicated the footer, and visually replaced the agent-name row with whatever stderr printed last. Confirmed by screenshot + OCR — the burned-in garbage matches the stderr text exactly.

The fix

Everything that can fire mid-session now goes through a diagnostic sink instead of stderr:

• PluginManager — hook warnings report through the sink
• BaseOrchestrator — SetDiagnostics/reportf, and the sink propagates to spawned children in the spawn runner
• tool package — bash-gate hints route through the sink

The TUI installs one that renders a warning toast via a new DiagnosticMsg (6s expiry, standard clear tick, truncated to terminal width with the status bar's ellipsis helper). stderr stays the fallback for headless/CLI runs, where there's no alt-screen to paint over.

reportf is guarded by its own diagMu instead of the orchestrator's main lock, so it stays callable while mu is held — the naive version deadlocked.

TUI fixes this surfaced

• Expiry-aware toast clear — clearToastMsg checks ToastExpireTime, so a clear tick scheduled by a previous toast can't kill a newer one early, and stale toasts can't be cleared twice.
• McpStatusMsg clears on a real 3s tea.Tick — the old immediate-clear command ran the moment Bubble Tea processed it, erasing the toast after a single rendered frame even though expiry said 3s.
• Global KeyReleaseMsg swallow — nothing consumes key-release events, so updateInternal drops them at the top; an esc release can no longer cancel a pending prompt.

Ported onto upstream/main's shapes: BaseOrchestrator carries only SetDiagnostics/reportf + diagMu (upstream has no dropped-events machinery), and the child sink installs in the actual spawn runner.

Testing

$ go test -count=1 -race ./...
ok  	late/cmd/late	2.097s
?  	late/cmd/mcp-run	[no test files]
ok  	late/internal/agent	1.869s
?  	late/internal/assets	[no test files]
ok  	late/internal/client	2.227s
ok  	late/internal/common	2.461s
ok  	late/internal/config	1.626s
ok  	late/internal/executor	9.919s
ok  	late/internal/git	2.950s
ok  	late/internal/mcp	2.932s
ok  	late/internal/orchestrator	3.215s
?  	late/internal/pathutil	[no test files]
ok  	late/internal/plugin	13.033s
ok  	late/internal/session	3.802s
ok  	late/internal/skill	3.169s
ok  	late/internal/tool	4.272s
ok  	late/internal/tool/ast	3.971s
ok  	late/internal/tui	9.057s

exit 0 — all 18 packages green (15 ok, 3 without test files).