fix(banner): prefer installed repo path in check_for_updates

[zerone0x]

Fixes #6319

check_for_updates() in hermes_cli/banner.py preferred ~/.hermes/hermes-agent over the project root. When a user installs from source at a different location and a stale clone lives at ~/.hermes/hermes-agent, hermes --version perpetually reported "N commits behind" even after updating the live checkout.

This patch resolves the repo directory against the project root (where __file__ lives) first, and only falls back to ~/.hermes/hermes-agent when the project root isn't a git checkout (e.g. pip-installed wheel). Minimal change, preserves existing caching and fetch behavior.

[alt-glitch]

Competes with #7303 and #4424 — same banner update-check path bug. Maintainer should pick one.

[zerone0x]

Thanks, confirmed the overlap:

• fix(cli): prefer active checkout for update banner #7303 is still open and covers the same active-checkout banner resolution path, with added regression coverage, though it also includes unrelated fixes.
• fix(cli): honor main tracking remote during update checks #4424 is still open and touches the same banner update-check area, but focuses on resolving the update remote/tracking branch rather than just choosing the active checkout path.
• This PR remains a minimal hermes_cli/banner.py-only variant for the stale ~/.hermes/hermes-agent checkout issue.

Since neither related PR is merged/closed as the chosen fix, I am leaving this open for maintainer selection. No code update is needed here unless maintainers want this variant adjusted.