feat(install): add --with-gbrain flag to install gbrain alongside fleet

[yashrajsapra]

Summary

• Adds --with-gbrain flag to apra-fleet install
• One-command setup: apra-fleet install --with-gbrain clones gbrain, runs bun install + bun link, and prints next steps
• Idempotent — skips gracefully if already installed, re-links if dir exists but CLI not in PATH
• Non-breaking — flag is opt-in, existing installs unaffected

Usage

apra-fleet install --with-gbrain
# Step 9: clones https://github.com/garrytan/gbrain → ~/gbrain
# Step 9: bun install + bun link
# Step 9: ✓ gbrain 0.33.1.0
# Next: run `gbrain init` to create your brain database.

Why this flag, not automatic

gbrain is opt-in (gbrain: true per member). Not every fleet user needs it. The flag makes the happy path easy without forcing a ~300MB download on everyone.

Failure modes handled

Scenario	Behaviour
bun not found	Warning + skip (non-fatal)
Already cloned + in PATH	Skip with "already installed"
Already cloned, not in PATH	Re-links via bun link
bun install postinstall fails (Windows)	Caught — benign, packages still installed

Test plan

• 5 new unit tests (all mocked — no network): bun-not-found, already-installed, clone, install+link, flag parsing
• All 12 install tests passing
• CI green

Related: part of the gbrain integration story (see PR #266)

🤖 Generated with Claude Code

[yashrajsapra]

Folding into #266 instead.