feat(cloud-agent): sandbox gh token refresh via refresh-session

[tatoalo]

Problem

agent-server's process env is frozen at launch, so its in-process tools (e.g. the signed-commit tool) kept
using the stale launch-time token after a rotation and nothing in the run log signalled that a refresh had happened.

Changes

• resolve the gh token from the live agentsh env file first, falling back to the process env, so in-process tools pick up
  a mid-session rotation without rebuilding the session (signed-commit, claude/codex adapters)
• short-circuit a credentials-only notification so it's safe mid-turn without rebuilding the session

[greptile-apps]

Prompt To Fix All With AI

Fix the following 3 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 3
packages/agent/src/utils/github-token.test.ts:15-25
**Prefer parameterised tests for the happy-path token-variable cases**

The two cases that exercise `GH_TOKEN` vs `GITHUB_TOKEN` precedence share the same structure and could be expressed as a single `it.each` table, keeping the distinct-behaviour tests (missing file, live-read, empty value) as individual `it` blocks. This also makes it easy to add new token variable names later without copy-paste.

### Issue 2 of 3
packages/agent/src/utils/github-token.test.ts:1-5
**`resolveGithubToken` — the public API — has no tests**

The test suite only exercises `readGithubTokenFromSandboxEnvFile` directly. `resolveGithubToken` adds an important second layer: it falls back to `readGithubTokenFromEnv()` (process env) when the sandbox file is absent. That fallback branch — the path taken in local/desktop environments — has no coverage, so a regression there (e.g. if the `??` chain were accidentally short-circuited) would go undetected.

### Issue 3 of 3
packages/agent/src/utils/github-token.ts:16
**Synchronous disk read on every tool invocation**

`readFileSync` blocks the Node.js event loop for the duration of the file read. Because `resolveGithubToken()` is called inside the async `signed-commit` handler (and similarly in `claude-agent` / `codex-agent`), every commit triggers a sync I/O operation. For a small `/tmp` file this is unlikely to be a problem in practice, but if token resolution is ever called from a hot path, consider switching to `fs.promises.readFile` and making the function async to stay non-blocking.

_{Reviews (1): Last reviewed commit: "feat(cloud-agent): sandbox gh token refr..." | Re-trigger Greptile}