fix(server): name the missing project folder instead of a provider spawn error

[EzraBuild]

Fixes #4940

Problem

If a project's folder is moved or deleted after it was added to T3 Code, the next turn fails with a raw provider spawn error (spawn ... ENOENT for Codex, and equivalents for the other providers) plus a stack trace in the toast. Nothing tells the user which folder went missing, so it reads like a broken provider install rather than a moved project.

Fix

ProviderCommandReactor now checks that the project's workspaceRoot still exists right before it spawns a new provider session. The check is skipped for threads that have a worktree, since their provider runs in the worktree under T3 home and ensureThreadWorktree already recreates a missing one. If the folder is gone, the turn fails with a ProviderAdapterRequestError whose detail is shown verbatim:

This project's folder no longer exists: /path/to/the/project

The check lives in the reactor, ahead of provider selection, so Codex, Claude, Cursor, Grok, OpenCode, and Antigravity all report the same cause instead of six different spawn failures. The session lands in the usual error state with lastError set and a provider.turn.start.failed activity, so the existing UI paths surface it with no client changes.

Re-linking a moved project to its new location, as the issue also suggests, is a separate feature and out of scope for this PR.

Tests

• Two new tests in ProviderCommandReactor.test.ts:
  • Deletes the project root, dispatches a turn start, and asserts the session error, the failure activity with the exact message, and that neither startSession nor sendTurn was called.
  • A worktree thread whose project folder was deleted still starts its provider in the worktree and sends the turn.
• The harness now creates a real project directory under the test state dir instead of the placeholder /tmp/provider-project path, so the existing tests keep passing the new check.
• Verified locally: vp test run on the reactor test file (60/60), vp lint, vp fmt --check, and the server typecheck.

Model: Claude Fable 5.1. Harness: Claude Code.

[macroscopeapp]

Approvability

Verdict: Would Approve

Macroscope's review found this PR approvable — This is a narrowly scoped server bug fix that adds a descriptive missing-folder error while preserving normal and worktree-backed session startup, with targeted tests covering both cases. An unresolved High-severity Macroscope finding is separately subject to the repository’s blocking threshold policy.

Not approved because:

• 1 blocking correctness issue found at or above your repo's Minimum Blocking Severity

^{Adjust the Minimum Blocking Severity for this repo — including turning it Off — in Settings. You can add or adjust custom eligibility rules. Learn more.}

[shivamhwp]

Note: GPT-6 on behalf of Shivam (@shivamhwp).

Closing as already covered by #5040. Provider startup now checks the workspace folder and returns an actionable missing-path error. Relinking a moved project remains tracked separately in #4940.