perf: reduce shell and worktree discovery process churn

[wsp1911]

Summary

• Cache selected shell validation and successful Node.js availability probes with runtime invalidation.
• Add a repository-scoped worktree topology cache shared across workspace and hook consumers.
• Avoid redundant worktree metadata refreshes during normal dialog activity.
• Harden worktree creation and frontend error reporting.
• Add a Windows process-event diagnostic probe.

Type and Areas

Type:

Performance / bug fix / test

Areas:

Rust core, terminal services, Git services, workspace services, desktop/Tauri, web UI, diagnostics

Motivation / Impact

Shell discovery, Node.js availability checks, and Git worktree topology queries could repeatedly create short-lived processes on Windows. This added unnecessary startup overhead and process noise during agent commands and workspace activity.

This PR makes shell and Node discovery lazy and reusable, coalesces concurrent probes, and invalidates cached results when the runtime changes or a process fails to spawn.

Worktree topology is now cached by the common Git directory with metadata-based freshness checks and explicit invalidation after mutations. Routine dialog activity only updates workspace activity timestamps instead of reloading Git metadata.

Worktree creation now rejects repositories without an initial commit before creating .worktrees, excludes the managed directory from Git tracking, resolves the newly created checkout directly, and distinguishes creation failures from workspace-opening failures in the UI.

Verification

Manual Windows checks:

• Started new agent sessions and ran echo 123 and ls with Git Bash, PowerShell 7, and Windows PowerShell.
• Repeated commands without changing the selected shell and confirmed cached discovery results were reused.
• Switched between Git Bash and PowerShell and confirmed the selected-shell cache refreshed as expected.
• Created and opened a linked worktree from an initialized repository successfully.
• Attempted worktree creation in an empty repository and confirmed it was rejected before creating .worktrees.
• Used scripts/diagnostics/probe-bitfun-process-events.ps1 to capture process start/stop events and verify BitFun process lineage.

Coverage added for:

• Shell cache reuse, invalidation, retries, and concurrent probes.
• Node.js availability cache refresh.
• Worktree topology cache sharing, metadata refresh, explicit invalidation, and concurrent requests.
• Worktree creation and unborn-repository rejection.
• Frontend create-stage and open-stage error classification.

Reviewer Notes

• Worktree topology entries use a five-minute TTL, metadata fingerprints, mutation invalidation, and a maximum of 64 cached repositories.
• Main and linked worktrees share one cache entry through their common Git directory.
• Explicit worktree listing requests force a refresh, while workspace and hook consumers use cached topology.
• Git worktrees remain unsupported for remote SSH workspaces.
• The process-event probe is Windows-only and intended for diagnostics.
• No new user-facing strings were introduced; existing localized error messages are reused.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.