fix(agents): keep session compactions branch-local

[ben-reitz]

Problem

Session compaction overlays* were selected globally before the provider checked whether their endpoint belonged to the requested conversation branch. A newer sibling-branch compaction could therefore hide a valid compaction for the branch being read.

SQLite compactions were also ordered only by CURRENT_TIMESTAMP, whose one-second resolution made the winning overlay nondeterministic when multiple compactions were created together.

• called an overlay because it's a summary of a single thread of messages through the tree. Those messages are still stored - the compaction is just an 'overlay' over the top of them.

Fix

• validate each compaction endpoint against the requested root-to-leaf path before selecting it
• select the latest valid branch-local overlay in both Agent and Postgres session providers
• order SQLite compactions by created_at, rowid for deterministic ties
• preserve iterative compaction on the same branch without allowing sibling branches to interfere

Testing

• 170 Session/provider tests
• includes sibling-branch and same-timestamp regressions for both providers

Stack

This PR was getting too big so I've stacked a bunch of related fixes into separate PRs:

1. This PR — Session branch-local compaction
2. fix(think): preserve regeneration branch context #2038 — Think regeneration context
3. fix(think): preserve active transcript during repair #2121 — transcript repair ownership
4. fix(think): preserve branches during recovery #2122 — branch-aware recovery
5. fix(think): preserve regeneration branches after restart #2057 — restart durability

[changeset-bot]

🦋 Changeset detected

Latest commit: 0028a20

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
agents	Patch
@cloudflare/agent-think	Patch

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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