fix(store): harden DB open against path injection (CodeQL #15)

[joshuaboys]

Closes CodeQL alert #15 (rust/path-injection, high): uncontrolled data used in path expression at crates/kindling-store/src/db.rs create_dir_all.

Investigation

This is a real sink (create_dir_all + SQLite open on a caller-supplied path) with a narrow threat model, not a classic remote traversal of X-Kindling-Project.

Source	Reaches open_database as	Risk
Daemon X-Kindling-Project	project_db_path(home, root) → home/projects/<12-hex SHA-256>/kindling.db	Already confined. The header is never a path component.
CLI --db / KINDLING_DB_PATH	Raw filesystem path (intended local config)	Operator can point the DB anywhere they can write. Accepted for a local-first tool.
Public SqliteKindlingStore::open / KindlingService::open	Same raw path	Library callers must not pass untrusted strings.

Two issues were not accepted residual:

1. SQLite URI injection. Readonly opens set SQLITE_OPEN_URI, and OpenFlags::default() includes URI as well. A path like file:/tmp/x?mode=rwc could be reinterpreted as a URI instead of a file path.
2. create_dir_all before validation. The CLI (open_service, init --skip-db) mkdir'd parents from the raw path, so a .. value could create directories even if open later failed.

Confining every open to ~/.kindling would break legitimate --db, KINDLING_DB_PATH, and temp-dir tests. The store therefore still accepts any local filesystem location without .. / URI / NUL.

Fix

• validate_db_path runs before mkdir or SQLite open. Rejects empty / non-UTF-8 / NUL, SQLite special names (file:…, :memory:, :temp:), and .. (the CodeQL DotDotCheck sanitizer guard). The path used at the sink is reconstructed from the checked string.
• Open flags drop SQLITE_OPEN_URI and add SQLITE_OPEN_NOFOLLOW.
• project_db_path documents and debug_asserts that project_id is a single 12-hex component.
• CLI no longer mkdir's before the store validates; init --skip-db validates first.

Residual

An operator can still open/create a DB at any path that does not contain .. (e.g. --db /var/tmp/kindling.db). That is intentional local configuration, not remote input. Daemon project routing remains hash-confined under kindling_home.

Tests

• Store: nested parents still created; .., URI/special names, empty, and NUL rejected.
• Paths: traversal-like project_root values stay under kindling_home as projects/<hex>/kindling.db.