feat(desktop): support manual context compaction in Side Conversations

[testikun]

Summary

• Support the exact /compact command in idle, committed Side Conversations.
• Route compaction to the companion Session without sending /compact to the model or touching the source Session.
• Keep draft, quote, and attachment state intact when the fork is preparing, running, archived, unavailable, or the Host rejects the operation.

Implementation

• Added the existing Session-scoped compaction operation to the Workbar Side Conversation port and Desktop adapter.
• Added command interception before steering/send and reused the existing compaction progress/terminal presentation semantics for the companion panel.
• Added retry-safe in-flight fencing, exact model/archive checks, and localized workspace-unavailable error handling.
• Fixed Runtime Host terminal projection so synthesized complete events preserve contextCompactionOutcome, allowing asynchronous Side Conversation compaction to settle and release its lock.

Tests

• Runtime Host projector regression: 20/20 passing.
• Desktop Side Conversation focused suite: 32/32 passing.
• Desktop typecheck: passing.
• Renderer architecture scanner: passing.
• ASF header and git diff --check: passing.
• Full workspace suite under Node 24: first run reached 1,566 passing / 12 skipped with one unrelated owned Host exits promptly after its first connection closes failure; the complete Runtime Host suite rerun passed 1,567 / 12 skipped.

Platform limitations

• This is a Desktop Workbar feature only; CLI/TUI /compact behavior is unchanged.
• It uses the existing Runtime Host context-compaction capability, so the operation remains subject to the selected model, workspace availability, and platform support already enforced by that capability.

Fixes #4521