Let skill status and remove forget missing installations

[openai0229]

What

sqlx skill status listed a record forever after its directory was deleted, always as intact: false, and there was no command to drop the record — the only fix was editing ~/.sqlx/skill-installs.json by hand.

• status now reports missing: true for a record whose directory no longer exists, so "deleted" and "locally modified" are distinguishable.
• New sqlx skill remove --path <skill-directory> drops the record. It never deletes files; the response reports directory_exists so the caller knows whether anything is still on disk.
• Path matching resolves the nearest existing ancestor before comparing, because the installer records the canonical path: after /tmp/... is deleted, remove --path /tmp/... still matches the recorded /private/tmp/... (this was a real failure found while testing the command).
• Docs: README command table and the Skill's status/update paragraph.

Verification

$ sqlx --data-dir … skill install --path /tmp/sqlx-loop/sk-test2
{"data":{"installed":true,"path":"/private/tmp/sqlx-loop/sk-test2","version":"0.1.6"},"success":true}
$ rm -rf /tmp/sqlx-loop/sk-test2
$ sqlx --data-dir … skill status
{"data":{"installations":[{"intact":false,"missing":true,"path":"/private/tmp/sqlx-loop/sk-test2","version":"0.1.6"}]},"success":true}
$ sqlx --data-dir … skill remove --path /tmp/sqlx-loop/sk-test2
{"data":{"directory_exists":false,"path":"/private/tmp/sqlx-loop/sk-test2","removed":true},"success":true}
$ sqlx --data-dir … skill status
{"data":{"installations":[]},"success":true}
$ sqlx --data-dir … skill remove --path /tmp/sqlx-loop/sk-test2
{"error":{"code":"sqlx.error","message":"no SQLX-managed Skill is recorded at /tmp/sqlx-loop/sk-test2; run {"data":{"installations":[{"intact":true,"path":"/Users/dawn/.pi/agent/skills/sqlx","version":"0.1.6"},{"intact":true,"path":"/Users/dawn/.agents/skills/sqlx","version":"0.1.6"}]},"success":true} to list them"},"success":false}

cargo fmt --all -- --check, cargo clippy --workspace --all-targets --locked -- -D warnings passed, and cargo test -p ottermind-sqlx runs three unit tests for removal, the missing flag and the deleted-directory path match.

Risks and limits

• remove intentionally does not delete files; removing a directory remains a user action.
• Only affects the CLI; the npx installer has no Skill-removal step.
• This branch and the dsh/pi target branch both append to crates/cli/src/skill.rs; whichever merges second needs the test modules combined.