chat: Cache generated built-in agent files

[roblourens (roblourens)]

What changed

• add a shared content-keyed writer for generated built-in agent files
• reuse concurrent and completed writes when generated content is unchanged
• clear failed writes so later provider calls can retry
• use the writer for the Plan, Ask, and Explore agent providers

Why

Custom-agent discovery can run repeatedly as workspace/session context changes. These providers regenerated identical content but rewrote their cache files on every call. In the Agents window, repeated session switching could therefore enqueue many identical writes and retain their buffers behind the serialized file-write queue.

This keeps provider discovery dynamic while making the generated-file I/O idempotent.

Validation

• Copilot extension type-check
• Copilot extension compile
• 55 focused provider/writer tests
• full repository hygiene

(Written by Copilot)

[Copilot]

Pull request overview

Adds idempotent caching for generated built-in agent files to avoid redundant writes.

Changes:

• Introduces a shared content-keyed file writer.
• Migrates Plan, Ask, and Explore providers.
• Adds deduplication coverage; failure-retry coverage remains needed.

Show a summary per file

File	Description
cachedAgentFileWriter.ts	Implements cached writes and failure eviction.
cachedAgentFileWriter.spec.ts	Tests successful write deduplication.
planAgentProvider.ts	Uses the cached writer for Plan.
askAgentProvider.ts	Uses the cached writer for Ask.
exploreAgentProvider.ts	Uses the cached writer for Explore.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 1
• Review effort level: Balanced