YM-195 Add pool model fallback

[prineycom]

Summary

• Resolve foreground and background mode models through explicit override, passport mode/default, then home/pool.json when no passport exists.
• Keep passport disagreement and legacy null values authoritative errors, and preserve first-ticket model selection for ordered ship batches.
• Default add-project to validated pool.do when no scalar --model is supplied.
• Expand diagnostics, regression coverage, bootstrap guidance, and model-pool documentation for all seven modes.

Verification

• pnpm test: 321 passed, 0 failed, 0 skipped.
• pnpm exec tsc --noEmit: passed.
• git diff --check origin/main...HEAD: passed.
• Independent re-review approved with no findings after restoring ordered ship PR coverage.

Integration notes

• Branch is 0 commits behind and 5 ahead of origin/main at opening.
• No changed-file overlap with open PR Isolate pnpm readiness installs #25.

Ticket: https://yoke.youtrack.cloud/issue/YM-195