[cherry-pick] Avoid Agent Host session listing starvation

[vs-code-engineering]

Cherry-pick of #333579 from release/1.136.

⚠️ This cherry-pick has merge conflicts that need manual resolution. Conflict markers (<<<<<<</=======/>>>>>>>) are committed to the branch — check it out locally, resolve, and force-push.

git fetch origin cherry-pick/333579 && git checkout cherry-pick/333579
# resolve conflicts, then:
git add -A && git commit --amend --no-edit && git push --force-with-lease

Conflicting files:

• src/vs/platform/agentHost/test/node/codex/codexModelRefresh.test.ts

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Summary

• avoid repeated Agent Host session-list recomputation under catalog invalidation
• persist the Recent-mode local-session cutoff without immediately reconciling active external sessions
• defer external-session SDK discovery until startup settles while preserving legacy migration discovery
• prevent Codex activation from starting unrequested external discovery

Addresses #333284.

Validation

• AgentService aggregation suite: 93 passing
• Codex model refresh suite: 50 passing
• targeted Claude, Copilot, Codex, and external-session discovery tests

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

TylerLeonhardt

Matched files:

• src/vs/platform/agentHost/node/claude/claudeAgent.ts

[Copilot]

Pull request overview

Defers external-session discovery and reduces session-list invalidation to prevent Agent Host listing starvation.

Changes:

• Defers provider discovery until startup settles.
• Persists Recent-mode local-session cutoffs.
• Avoids invalidation for idle provisional sessions and updates tests/documentation.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/chat.shared.contribution.ts	Updates Recent-mode description.
src/vs/platform/agentHost/test/node/copilotAgent.test.ts	Explicitly starts Copilot discovery.
src/vs/platform/agentHost/test/node/codex/codexModelRefresh.test.ts	Tests Codex discovery behavior; contains unresolved conflicts.
src/vs/platform/agentHost/test/node/claudeAgent.test.ts	Explicitly starts Claude discovery.
src/vs/platform/agentHost/test/node/agentService.test.ts	Covers discovery, cutoff persistence, and invalidation.
src/vs/platform/agentHost/node/copilot/copilotAgent.ts	Exposes explicit discovery startup.
src/vs/platform/agentHost/node/codex/codexAgent.ts	Tracks host-requested discovery across activation.
src/vs/platform/agentHost/node/claude/claudeAgent.ts	Exposes explicit discovery startup.
src/vs/platform/agentHost/node/agentService.ts	Coordinates deferred discovery and persisted recency state.
src/vs/platform/agentHost/common/agentHostSchema.ts	Updates Agent Host setting description.
src/vs/platform/agentHost/common/agent.ts	Adds the discovery-start API.
src/vs/platform/agentHost/AGENTS.md	Documents the revised discovery lifecycle.

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[github-actions]

Base: 3e33b210 Current: 06badc5e

No screenshot changes.