fix: permission issues in sandboxes

[stephantul]

This PR correctly adds the distinction between a config file not existing and a config file not being readable because of other issues. Previously, we assumed that if a config file existed, it was also readable, but this is not true in the case of sandboxes, which caused errors.

Solves #255

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/semble/installer/agents.py	100.00% <100.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge with no actionable defects identified.

The permission-aware helper preserves the established OpenCode configuration precedence and intentionally makes best-effort agent detection tolerate sandbox-denied metadata access.

_{Reviews (1): Last reviewed commit: "fix: permission issues in sandboxes" | Re-trigger Greptile}