v0.8.0 - Cancellation reaches every external call

Compared against v0.7.0 (73f4aac).

Published crate bumps:

• halter-protocol 0.7.0 -> 0.8.0
• halter-hooks 0.7.0 -> 0.8.0
• halter-config 0.7.0 -> 0.8.0
• halter-providers 0.7.0 -> 0.8.0
• halter-session 0.7.0 -> 0.8.0
• halter-tools 0.7.0 -> 0.8.0
• halter-runtime 0.7.0 -> 0.8.0
• halter 0.7.0 -> 0.8.0

Changes:

• Cancellation reaches every external call a turn makes: provider HTTP/SSE, rate-limit waits, model-judge panels, hooks, subagent spawn/wait and browser actions.
• submit_turn_with_cancel (now public) and compact_with_cancel let callers cancel a turn or compaction under their own CancellationToken, including while waiting for the session's write lease. submit_turn, compact and notify stop at runtime shutdown. Dropping a turn's event stream still does not cancel it.
• Fixes:
  • Cancelled rate-limit waits, body reads and stream setup surface as cancellations, not fatal or retryable errors.
  • The provider stream's cancel marker is no longer dropped when the event channel is full.
  • A cancelled model-judge turn no longer falls back to the default model.
  • Command hooks kill their child on timeout.
  • Agent hooks cancel their sub-turn on timeout or drop.
  • A turn cancelled between rounds stops instead of starting another round.
• The Browserbase, GitHub plugin fetcher and CLI OAuth HTTP clients have connect and request timeouts.

Breaking:

• SubagentControl::spawn and SubagentControl::wait take a CancellationToken.
• HookInput has a new cancel field.

Known remaining gaps: #211.