feat: add adopt command to move existing files into the overlay

[lollipop-onl]

Summary

Setting up a new overlay file today is manual: mkdir the overlay entry
directory, cp/mv the file into it, then run ghostq apply. This friction
discourages moving existing personal files into the overlay after the fact,
even though the overlay is meant to be the durable source of truth for
gitignored, per-repo files.

ghostq adopt <file>... collapses this into one step per file:

• Resolve the file's repo context via resolveContext (same identity
  resolution as apply/status), rejecting files outside a git repo or in a
  repo with no resolvable remote identity.
• Move the real file into the mirrored path under the repo's overlay entry
  (fs.rename, with a copy+unlink fallback for EXDEV when the overlay lives
  on a different filesystem).
• Symlink the original checkout path back at the moved file — the same shape
  of link apply produces, so a subsequent ghostq apply on that file is a
  no-op (alreadyLinked), verified directly in tests.

Safety guarantees

Per invariants in AGENTS.md:

• Gitignore-gated: a target that fails git check-ignore is skipped with
  a warning — adopt never moves a tracked file into the overlay.
• Never clobber: if the overlay already has a file at the target path, or
  the checkout path is already a symlink pointing somewhere else, the file is
  skipped and reported, not overwritten.
• Already-adopted is a no-op: a file already symlinked into this repo's
  overlay entry is reported as already-adopted rather than re-processed.
• symlink-each, never whole directories: passing a directory is rejected
  with a message to pass individual files instead, matching how apply links
  files individually so committed and overlay-linked files can sit side by
  side in the same directory.
• All new logic lives in src/adopt.ts and returns a data structure per file
  (adopted / already-adopted / skipped-not-ignored / skipped-conflict
  / skipped-directory / error); only src/index.ts prints, consistent with
  the rest of the codebase.

Test plan

• bun install
• bun test — 42 pass (adds test/adopt.test.ts covering: move +
  symlink + content preservation, idempotency bridge with applyFiles,
  tracked-file skip, already-adopted no-op, overlay-conflict skip,
  foreign-symlink skip, directory rejection)
• bun run typecheck
• bun run build

🤖 Generated with Claude Code

https://claude.ai/code/session_013mAzjNvVWGBJT81cfTSJJN