fix: bump better-sqlite3 for Node 24 prebuilds and fail gracefully without the native module

[brainsparker]

Why

Two related problems with the SQLite dependency:

1. better-sqlite3 ^11.0.0 has no prebuilt binary that resolves on Node 24, so npx youagent fails to install on machines without a C++ toolchain. Verified: npx youagent@0.0.1 exits 1 on Node 24 in a clean environment because the postinstall build needs make and g++.
2. When the native module is missing or built for a different Node version, commands that touch the database (feed, follow, ask) crash with a raw bindings stack trace instead of telling the user what to do.

What changed

• package.json: better-sqlite3 ^11.0.0 to ^12.4.1 (resolves to 12.11.1 in the lockfile). v12 ships prebuilds for current Node versions including 24. v13 is out too; I kept the smaller jump, happy to go to 13 if preferred.
• src/storage/database.ts: the AgentDatabase constructor now detects native-module load failures (ERR_DLOPEN_FAILED, MODULE_NOT_FOUND, bindings file messages, NODE_MODULE_VERSION mismatches) and throws a clear error with remediation steps (npm rebuild better-sqlite3, or reinstall on Node 20+). Ordinary database errors are rethrown unchanged.

Validation

• npm run build passes (tsup, ESM)
• tsc --noEmit passes
• Test suite passes: 48/48
• Manually triggered the failure path with the native module absent: feed now reports the actionable message instead of the raw bindings trace
• Could not verify the Node 24 prebuilt download end to end in this environment (no C++ toolchain to compare against a source build), so worth one npx youagent smoke test on a normal machine after the next publish

Follow-up worth considering

The published youagent@0.0.1 predates recent repo work. After merging, publishing a fresh version would pick up both fixes for npx users.