Stop a missing C++ toolchain from breaking the install - #28

Closes #27

Storage now uses Node's built-in node:sqlite, which needs no compiler. better-sqlite3
moves to optionalDependencies, so a failed native build warns instead of failing the
install, and it is only ever loaded on a Node too old for the built-in.

This is the fix docs/PUBLISHING.md already recommended for this exact problem.

Verified before writing any code

Against Node's own doc/api/sqlite.md history blocks and runtime probes on a real machine —
not from memory:

iterate() landing in exactly the release that removed the flag is why the floor is one
boundary rather than two, and why the capability probe in the driver tests for iterate()
rather than sniffing versions.

Live test against a real 879 MB OpenCode database (WAL, with a 30 MB -wal) — the
"don't disturb a running OpenCode" case:

YES  open readOnly                      YES  journal_mode {"journal_mode":"wal"}
YES  count messages {"c":7791}          YES  iterate
YES  writes rejected (OpenCode undisturbed)

.db and -wal byte-identical afterwards; only -shm's mtime moved, which is SQLite's
shared-memory coordination and is what better-sqlite3 does too.

Design

Everything goes through openSqlite() in the new src/db/driver.ts — no other module
imports a SQLite driver directly, which is what keeps the fallback honest. Recorded as an
invariant in CLAUDE.md.

node:sqlite has no .pragma() or .transaction(), so the driver supplies both. Transactions
nest via savepoints, matching better-sqlite3, because migrate() runs each migration inside a
transaction and an inner BEGIN would otherwise throw.

changes is coerced to a number: node:sqlite can return a bigint there, and the repositories
do arithmetic on it.

Invariants

verify through both drivers — identical figures, zero delta, against the same real
usage.db:

AI_USAGE_SQLITE_DRIVER=node:sqlite      -> 3x MATCH, RESULT: every client reconciles exactly
AI_USAGE_SQLITE_DRIVER=better-sqlite3   -> 3x MATCH, RESULT: every client reconciles exactly

File format untouched, no migration. The existing 12,599-record database — written by
better-sqlite3 — reads correctly under node:sqlite: schema_migrations intact,
journal_mode=wal, real rows readable. Header is plain SQLite format 3, page size 4096,
because the format is SQLite's, not the binding's.

npm run check passes: 151 tests (143 + 8 new). parity.test.ts untouched and passing.

The plan's "Done when": a clean install with no build step

npm install <tarball> --ignore-scripts   -> exit 0, no native binding built, SQLite driver: node:sqlite
npm install <tarball> --omit=optional    -> exit 0, better-sqlite3 absent entirely, SQLite driver: node:sqlite

Full MCP handshake in the --omit=optional install: initialize OK, tools/list → 7 tools,
tools/call usage_summary OK. stdout stayed clean JSON-RPC; the ExperimentalWarning goes to
stderr, as it must.

One difference deliberately not papered over

node:sqlite returns rows with a null prototype; better-sqlite3 returns ordinary objects.
Property access, spreading and JSON.stringify are identical, but row instanceof Object is
false and row.hasOwnProperty is undefined. Nothing reads rows that way today (audited), so
rather than wrap every row and pay for it, the driver documents the constraint and a test pins
the difference.

Notes on scope

• engines.node moves >=22.0.0 → >=22.13.0. Phase 5 said not to touch the floor, but
  the stated "Done when" requires node:sqlite to be present, and below 22.13.0 it isn't —
  the native module would be mandatory again. Still "22+", just patch-precise. Flagging it
  because it narrows support to 22.13+ within the Node 22 LTS line.
• engines.npm dropped, and CI no longer upgrades the runner's npm — so an npm 10 Windows
  install is now a genuine regression test rather than a configuration we avoid.
• AI_USAGE_SQLITE_DRIVER forces either driver. Not in the plan; added because otherwise
  the fallback is untestable on a Node that doesn't need it, and it makes a suspected driver
  difference diagnosable.
• ai-usage status now prints SQLite driver:. With two possible drivers, which one
  produced the numbers is provenance, and this project already reports schema version and
  pricing provenance there.
• Windows npm notes in the README were reworded to apply to the fallback, not deleted.