fix(cli): honor HAPI_HOSTNAME in session metadata

[Shujakuinkuraudo]

Summary

This small fix makes session metadata use HAPI_HOSTNAME the same way machine metadata already does.

Problem

Today HAPI_HOSTNAME affects machine metadata, but session metadata still records os.hostname() directly.
That can be confusing in self-hosted setups where multiple clients share the same system hostname but are intentionally distinguished with HAPI_HOSTNAME.

Change

• use process.env.HAPI_HOSTNAME || os.hostname() in buildSessionMetadata()
• add a focused test covering the env override

Why

This keeps machine host and session host consistent, and improves host-based display / fallback behavior without changing default behavior for users who do not set HAPI_HOSTNAME.

[github-actions]

Findings

• None.

Summary
Review mode: initial. No diff-introduced correctness, security, regression, or maintainability issues found in the modified lines. Residual risk: this change only adds a focused env-override test, and fallback behavior to os.hostname() was not re-verified in automation here.

Testing

• Not run (automation): bun is not installed in this runner, so I could not execute cli/src/agent/sessionFactory.test.ts.

HAPI Bot