fix(runtime): keep host paths out of opencode acp directories

[FradSer]

Summary

Keep Host provisioning paths separate from optional ACP session directories for OpenCode (acp-fallback). Continue creating the cache, home, and working directories on the Host while sending no optional directories to this provider.

Why

The API sends Host cache/home/cwd paths as additionalDirectories. OpenCode does not advertise the optional ACP directory capability, so the Driver rejects the execution spec before starting a session. Rename the internal path helper to describe provisioning and send an empty optional-directory list for OpenCode. The session cwd and native home remain present in their existing fields.

Related history: Mosoo #402 and Driver #86 address the same OpenCode capability boundary. Extracted from #693 following review request 6. No dedicated current open issue was found, and no issue is closed here.

Verification

• just test-file apps/api/tests/api-driver-boundary.test.ts: 14 passed, including an OpenCode spec with no optional directories and preserved cwd/home.
• just test-file apps/api/tests/runtime-model-connection.test.ts: 44 passed.
• just tc-package @mosoo/api; just lint; just commit-check: passed.
• Full pre-review gate: just check passed on this exact head in an isolated local Linux environment (Bun 1.4.0, Node 24.22.0): Driver 2,282 passed / 47 skipped / 0 failed, API 1,174 passed / 1 skipped / 0 failed. Formatting, docs, lint, type checks, GraphQL freshness and Public API compatibility also passed.
• Full hosted CI for this head was triggered by marking the PR ready for review; see the run for its current result and native Host checkpoint step.
• Not run: live inference against a real OpenCode provider credential.

Review

• Closest review area: the API-to-Driver execution spec boundary.
• Env/config changes: N/A.
• Known trade-offs: deliberately scoped to OpenCode; other runtime behavior is preserved. No Pi RPC special case, Driver code, contract change, or Driver gitlink update.

Impact

• Generated files / GraphQL / DB / lockfile: N/A; unchanged.
• Risk and rollback: revert this isolated commit. No deployment or migration.

Design

N/A: no product UI changes.