fix(agents): correct Cloud VM bun PATH instructions in AGENTS.md

[Drswith]

Summary

Fix incorrect Cloud VM instructions in AGENTS.md. The previous text stated bun v1.3.11 was pre-installed via ~/.bun/bin, but in practice bun may not exist on fresh VMs and ~/.bashrc is not sourced in non-interactive shells. Updated to document the auto-install fallback in the update script and the explicit PATH export requirement.

Also updated the VM update script to:

1. Export BUN_INSTALL and PATH explicitly (non-interactive shells don't source ~/.bashrc)
2. Auto-install bun if not found
3. Run bun install

Linked Artifacts

• Issue: N/A (environment setup fix)
• ADR: N/A
• OpenSpec: N/A (no behavior/contract change)
• Discussion: N/A

Validation

• bun run memory:check
• bun run lint
• bun run format:check
• bun run typecheck
• bun run test (471 tests passed)

Release Intent

• Release: not applicable - docs/process/test-only change

Docs Updated

• Not needed (only AGENTS.md cloud instructions corrected)

Scope Check

• I did not add a new ad hoc root-level Markdown file.
• I updated the relevant issue, ADR, spec, runbook, or captured the missing doc work as follow-up.
• I did not silently expand project scope without recording it explicitly.

Closure Check

• Working tree was clean after commit.
• Branch was pushed and this PR is the active delivery artifact.
• OpenSpec change is not needed, still active by design until merge, already archived, or queued for agent-driven archive closure.
• Release is not applicable, delegated to release automation, or verified.