docs: add USE-CASES.md (situation-first recipes)

[srnnkls]

Summary

Adds USE-CASES.md, a third documentation angle alongside the README (reference, #10) and GUIDE.md (walkthrough, #11): each section starts from a situation and shows a working config for it.

Covered:

• Dotfiles — slices per ~/.config target, machine differences via phora.local.toml, the --symlink dev loop, and an explicit Where phora stops note (no templating, no hooks, copies not symlinks) with a fair pointer to dotter/chezmoi for setups that lean on those.
• Agent artifacts — skills/subagents/prompts pinned per project, per-project include narrowing, by-source bundles, link-mode skill development, verify in CI.
• Shared configuration across repositories — per-repo locks as the rollout mechanism, CI conformance via verify, side-by-side current/next versions via per-binding refs; nods to vendir as the same instinct.
• Release assets — URL sources vs curl | tar, digest-before-extraction, auto-strip, no lock churn on unchanged bytes.
• Subtree vendoring — protos/schemas/tokens from a larger repo, pinned and upgradable per consumer.
• A short other shapes list (git hook files, runbooks, reference checkouts).

Notes

• All config snippets follow phora.example.toml/GUIDE semantics; artifact-lands-as-directory is respected in every layout example.
• The GUIDE.md link resolves once docs: add GUIDE.md (extensive walkthrough + internals) #11 merges; the tests/scrut/showcase.md link is already on main.