fix: Queue messages during conversation compaction

[charlesvien]

Problem

Messages sent during compaction bypass the queue and are lost because isCompacting was only a derived UI property, invisible to the service layer.

Closes #1272

Changes

1. Add isCompacting state to AgentSession and gate sendPrompt on it
2. Track compaction lifecycle via STATUS and COMPACT_BOUNDARY notifications in service
3. Drain queued messages when compaction completes
4. Replace all _posthog/ string literals with POSTHOG_NOTIFICATIONS constants
5. Add isNotification() utility in agent package to handle potential double-prefix
6. Remove duplicate isPosthogMethod helpers from renderer files

How did you test this?

Maually

[charlesvien]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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