fix(start): preflight agent launch setup

[michiosw]

Summary

• Preflights the requested agent before auth, backend session creation, provider sync, or hosted connect.
• Distinguishes unsupported agents, missing binaries, non-executable binaries, and agent process exits after setup.
• Keeps downstream agent failures visible instead of silently forwarding only the exit code.

Why

If Claude is missing or cannot run, the CLI should say that before doing any auth or provider work. Developers get a faster failure and avoid creating managed sessions for a launch that was never possible.

Before / After Terminal Capture

Before, setup could run before discovering Claude was unavailable or invalid:

✓ Authenticated as dev@example.com
✓ Session: MacBookPro - kontext-cli (73ec64f7)
Resolving credentials...
Launching claude...
Error: agent "claude" not found in PATH: executable file not found

After, agent setup fails immediately:

Error: agent "claude" not found in PATH

For an agent that launches and exits early, the user gets Kontext context:

Launching claude...
Error: claude exited with code 1 after Kontext setup completed
✓ Session ended (73ec64f7)

Verification

• Ran go test ./... on this branch.

[michiosw]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(hooks): quiet default allow reasons #71
• fix(start): summarize missing provider setup #70
• fix(auth): reject gateway reauth account mismatch #69
• feat(cli): add verbose diagnostics mode #68
• fix(start): preflight agent launch setup #67 👈 (View in Graphite)
• fix(auth): use stable session identity #66
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[tumberger]

lgtm