fix(workspace): hold every resolved workspace exclusively

agent-, project-, and company-fallback workspaces resolved to
`shared`, so four agents sharing one repo wrote concurrently and
corrupted both the working tree and git's own index state before
the per-file `index.lock` even got a chance to kick in. The
previous comment justified this as "the normal case", but pi's
`createCodingTools()` hands every agent bash/write/edit — there is
no read-only category left where shared is safe.

This promotes every resolved workspace to `exclusive`. Contention
is still handled at the one place it must be: `run-dispatch.ts`
synchronously claims the workspace lock, and `finishRun`'s
never-started branch rolls the task back to `todo` on a lost
race. The "shared mode prevents deadlocks" story was really
"shared mode prevents one specific bug in the checkout rollback
path that has since been fixed"; the right invariant is single
writer per repo, enforced by the DB lock. Concurrent heartbeats
for the same workspace now queue through the wakeup table on the
next cycle rather than racing the filesystem.

src/main/orchestration/resolve-workspace.ts

@@ -2,34 +2,28 @@ import type { AppDatabase } from "../database";
 import type { WorkspaceLockMode } from "./types";
 
 /**
- * Resolve the workspace and lock semantics for a given run context.
+ * Resolve the workspace and lock semantics for a run context.
  *
- * Lock policy (v0.4.168+):
+ * Lock policy: every resolved workspace is held exclusively for the
+ * duration of the run. Pi's `createCodingTools()` hands every agent
+ * the mutating `bash`, `write`, and `edit` tools, so there is no
+ * read-only category where concurrent runs are safe — two agents
+ * editing the same working tree step on each other's files and
+ * corrupt git state before the per-repo `index.lock` even gets
+ * touched.
  *
- *   task.workspaceId set  → exclusive
- *     The user (or an upstream caller) explicitly bound a task to a
- *     workspace, which is the opt-in signal for "this run needs the
- *     workspace to itself". Rare, and reserved for workflows like a
- *     sprint merge or release build where concurrent writes would
- *     corrupt state.
+ * Contention is handled at dispatch time: `run-dispatch.ts` tries
+ * `claimWorkspaceLock` synchronously. A lost race finishes the run
+ * as `cancelled` (not `failed` — the Dashboard's "Needs attention"
+ * card ignores cancellations), and the next heartbeat retries on
+ * its own cadence. The caller must keep that rollback path alive;
+ * without it contention would leave tasks pinned at `in_progress`
+ * forever.
  *
- *   agent.workspaceId set   → shared
- *   project.workspaceId set → shared
- *   company fallback        → shared
- *     Multiple agents sharing the *same* workspace is the normal case
- *     — it represents "this team works in this repo", not "this agent
- *     owns this repo". The previous exclusive default forced four
- *     co-located teammates to run one at a time, which deadlocked
- *     heartbeats (dispatch bailed → task stuck in_progress → retry
- *     lost the checkout race forever). Git itself serializes the
- *     dangerous operations (`git index.lock`); anything coarser here
- *     just blocks readers.
- *
- * Previously this logic lived both in run-dispatch.ts (normal dispatch
- * path) and worker-crash-recovery.ts (re-enqueue after crash); the
- * recovery copy silently dropped the exclusive/shared distinction.
- * Sharing the helper guarantees both paths agree on which runs hold
- * exclusive locks.
+ * When no workspace can be resolved (headless heartbeat runs that
+ * have no repo to touch) we return `none`; the worker uses
+ * `process.cwd()` as a read-only cwd and the dispatcher skips the
+ * lock claim.
  */
 export function resolveWorkspaceWithLockMode(
   db: AppDatabase,
@@ -50,17 +44,17 @@ export function resolveWorkspaceWithLockMode(
 
   const agentWorkspace = db.getWorkspace(agent.workspaceId);
   if (agentWorkspace) {
-    return { workspace: agentWorkspace, lockMode: "shared", source: "agent" };
+    return { workspace: agentWorkspace, lockMode: "exclusive", source: "agent" };
   }
 
   const projectWorkspace = db.getProjectPrimaryWorkspace(task?.projectId ?? null, companyId);
   if (projectWorkspace) {
-    return { workspace: projectWorkspace, lockMode: "shared", source: "project" };
+    return { workspace: projectWorkspace, lockMode: "exclusive", source: "project" };
   }
 
   const fallbackWorkspace = db.getAnyCompanyWorkspace(companyId);
   if (fallbackWorkspace) {
-    return { workspace: fallbackWorkspace, lockMode: "shared", source: "company_fallback" };
+    return { workspace: fallbackWorkspace, lockMode: "exclusive", source: "company_fallback" };
   }
 
   return { workspace: null, lockMode: "none", source: "none" };