test: use --timeout CLI flag for model-based test timeouts (#367)

## Problem

Several model-based tests using `fast-check` intermittently timeout with
the default 5000ms limit:

- `test/lib/db/model-based.test.ts` — "alias lookup is case-insensitive"
- `test/lib/db/pagination.model-based.test.ts` — "get on empty table
returns undefined", "composite key: different command keys are
independent"
- `test/lib/db/dsn-cache.model-based.test.ts` — "random sequences of
cache operations maintain consistency"

### Root Cause

`bunfig.toml`'s `timeout = 30000` under `[test]` is **not a supported
config key** in Bun 1.3.x — it's silently ignored. All tests were
running with Bun's default 5000ms per-test timeout.

Model-based tests routinely take 5–17s per test due to 50 fast-check
iterations, each creating a new SQLite DB (12 tables + migrations) with
`size: "+1"` generating 10–30+ command sequences.

### Changes

1. **`bunfig.toml`** — Remove non-functional `timeout = 30000`
2. **`package.json`** — Add `--timeout 15000` to `test:unit`,
`test:isolated`, and `test:e2e` scripts
3. **`AGENTS.md`** — Document the bunfig.toml timeout gotcha as a lore
entry

Fixes #366

Author: BYK