[roblourens]

In remote agent host worktree mode, the generated branch was just a GUID because the eager-create path never sent a branchNameHint. Rather than plumb a hint from the client, this PR derives it on the agent host from the first message itself.

Changes

• Add getCopilotBranchNameHintFromMessage in copilotAgent.ts
• Plumb prompt through sendMessage → _materializeProvisional → _resolveSessionWorkingDirectory so the branch is created lazily on first message with a meaningful name
• Remove SessionConfigKey.BranchNameHint and the client-side getAgentHostBranchNameHint plumbing — the client no longer needs to compute or send a hint
• Update tests accordingly

[Copilot]

Pull request overview

This PR improves the remote agent host “worktree isolation” flow by deriving a deterministic branch-name hint from the first user message on the agent host, avoiding GUID-only branch names without requiring the client to compute or send a hint.

Changes:

• Add getCopilotBranchNameHintFromMessage on the agent host and use it during provisional-session materialization to name worktree branches based on the first prompt.
• Remove client-side branchNameHint plumbing (SessionConfigKey.BranchNameHint + handler helper/export) and adjust session config forwarding behavior/tests.
• Update UI/config picker and tests to reflect the removed config key and new eager-create config propagation.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostChatContribution.test.ts	Updates tests to stop expecting branchNameHint in createSession config and adds coverage for eager-create SessionConfigChanged forwarding.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts	Removes client-side branch hint derivation and stops injecting BranchNameHint into session creation config.
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostChatContribution.ts	Removes re-export of the deleted getAgentHostBranchNameHint.
src/vs/sessions/contrib/chat/browser/agentHost/agentHostSessionConfigPicker.ts	Stops filtering out the removed branchNameHint config key when rendering pickers.
src/vs/platform/agentHost/test/node/copilotAgent.test.ts	Updates tests to reflect prompt-based branch hinting and the removal of branchNameHint in config.
src/vs/platform/agentHost/node/copilot/copilotAgent.ts	Introduces prompt-to-slug branch hint derivation and plumbs prompt into provisional materialization/worktree creation.
src/vs/platform/agentHost/common/sessionConfigKeys.ts	Removes SessionConfigKey.BranchNameHint.

Copilot's findings

• Files reviewed: 7/7 changed files
• Comments generated: 1