Move headless permission setup out of the workspace allocator

[FreshlyBrewedCode]

Parent

#32

What to build

The agent runtime prepares its own workspace, so lib/workspace.ts goes back to being about git.

Today allocateWorkspace calls writeHeadlessPermissions, which writes an opencode.json
carrying permission: {"*": "allow"} into every run tree and registers it in the clone's
.git/info/exclude. That policy exists for a reason that is entirely opencode's: opencode asks
permissions that nobody can answer headless, and the adapter cannot even see the asks, so one ask
is a permanent silent deadlock of the run, its WIP slot and its workspace (#24). A workspace
allocator has no way to know that and no business knowing it.

After this ticket the agent runtime carries a workspace-preparation responsibility, and the
opencode adapter is what writes opencode's config. lib/workspace.ts keeps mirror, clone, identity
and retention — and nothing else. resetClone, which writes the same file on the legacy
single-clone path, moves with it.

The behaviour must not change: #24 was a real deadlock found in production, and this ticket is a
relocation, not a reconsideration. The existsSync guard in allocateWorkspace — which exists
only because workspace.test.ts's injected fake exec spawns nothing and so leaves dir absent —
should disappear along with the responsibility.

See docs/adr/0012-agent-runtime-seam.md §3.

Acceptance criteria

• lib/workspace.ts and lib/clone.ts contain no opencode-specific knowledge
• Workspace preparation is a responsibility of the agent runtime, invoked for both the per-run clone path and the legacy resetClone path
• A clone workspace still ends up with opencode.json and the matching .git/info/exclude entry before the first agent step runs
• writeBack still never stages opencode.json
• A scratch workspace behaves exactly as it does today
• The existsSync guard added for the injected-fake-exec case is gone
• bun run check passes

Blocked by

• Move chunk interpretation into the agent adapter #35 — the adapter interface reshape lands first, so the workspace-preparation responsibility is added to a settled contract.