agentHost: support file completions across workspace roots

[DonJayamanne]

Summary

For #321651

• provide agent-host file completions across all effective session and chat working directories
• collapse nested roots to a minimal enumeration set while preserving logical-root attribution, fair ordering, URI deduplication, and collision labels
• preserve host ranking and path filtering in both chat surfaces, and avoid incorrect worktree rebasing for nested secondary roots
• bound ripgrep enumeration concurrency and retain successful results when individual roots fail

Testing

• npm run typecheck-client
• npm run valid-layers-check
• targeted agent-host completion, workspace-file, chat-input, and worktree-rebase unit tests
• hygiene and diff checks

Here's what it looks like now

[Copilot]

Pull request overview

Extends agent-host file completions across effective session and chat workspace roots.

Changes:

• Adds multi-root enumeration, deduplication, ranking, and failure isolation.
• Preserves attachment ownership during worktree rebasing.
• Adds completion filtering, ordering metadata, documentation, and tests.

Show a summary per file

File	Description
chatInputCompletions.test.ts	Tests filtering and ordering metadata.
agentHostChatContribution.test.ts	Tests nested-root rebasing.
chatInputCompletionUtils.ts	Adds resource filter text helper.
agentHostInputCompletionsBase.ts	Assigns completion ordering metadata.
agentHostInputCompletions.ts	Enables absolute-path filtering.
agentHostSessionHandler.ts	Restricts rebasing to the owning root.
SESSIONS.md	Documents completion behavior.
sessions/.../agentHostInputCompletions.ts	Applies filtering in Agents Window.
agentHostWorkspaceFiles.test.ts	Tests caching and nested enumeration.
agentHostFileCompletionUtils.test.ts	Tests root planning and attribution.
agentHostFileCompletionProvider.test.ts	Covers multi-root provider behavior.
agentService.ts	Supplies logging to the provider.
agentHostWorkspaceFiles.ts	Limits concurrency and propagates failures.
agentHostFileCompletionUtils.ts	Computes logical and enumeration roots.
agentHostFileCompletionProvider.ts	Implements multi-root ranking and labels.
agentHostWorkingDirectories.ts	Resolves the most-specific owning root.

Review details

• Files reviewed: 16/16 changed files
• Comments generated: 4
• Review effort level: Balanced