fix(zq): stop defaulting the database path into the repo being deleted

`zq.py` hardcoded one absolute path, and it pointed inside `xenia-rs` --
a repository CONSOLIDATION.md archives in Phase 5 and drops in Phase 7. It
resolved on exactly one machine, and on that machine it was days from becoming
a `duckdb` exception about a missing file, with nothing saying why.

Resolution order is now `$SYLPH_XEXDB`, else `<repo root>/sylpheed.db`, else a
refusal that names both. The repo-root path is taken relative to this script,
not the caller's cwd, because zq.py is run from wherever the investigation is.

No fallback beyond that, on purpose. The database is a build artefact of a few
hundred MB and is not tracked, so there is nothing to fall back *to*; a default
that silently resolves to the wrong database is worse than no default. That is
issue #16's lesson, which was about exactly this shape in the test suite.

Both failure paths say what to do and exit 1:

  $ zq.py fn 82000000            # nothing set, nothing at the repo root
  no database found.
    looked for: /…/Sylpheed/sylpheed.db
    set $SYLPH_XEXDB to an existing one, or build it with:
      sylph-xexdb dis <xex|iso> --db sylpheed.db --analyze sql

  $ SYLPH_XEXDB=/nope/missing.db zq.py fn 82000000
  $SYLPH_XEXDB is set but is not a file:
    /nope/missing.db

Also fixes the regeneration hint, which still named `xenia-rs dis` -- the
retired binary. It is `sylph-xexdb dis`.

Connecting is skipped when there is no subcommand, so `zq.py` still prints its
usage on a machine that has no database yet.

Verified: usage with no db (rc 0); missing db (rc 1); `$SYLPH_XEXDB` set to a
non-file (rc 1); and `$SYLPH_XEXDB` pointed at a real 336 MB database, where
`imp Rtl` returns its 1,280 `RtlLeaveCriticalSection` call sites. That database
turns out to have been generated from the uncommitted tree this branch ports:
it carries `import_address`, `import_role` and all 16 foreign keys -- an
independent confirmation of the schema the golden test now locks.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/zq.py

@@ -38,18 +38,59 @@ Usage:
 
 A command that needs a table the current DB predates prints what to regenerate
 rather than a SQL error.
+
+The database is found via `$SYLPH_XEXDB`, else `<repo root>/sylpheed.db`. It is a
+build artefact, not a tracked file; if neither exists, zq.py says so and prints
+the command that builds one.
 """
-import duckdb, sys, signal
+import duckdb, sys, os, pathlib, signal
 
 # Output is routinely piped into `head`; without this Python prints a
 # BrokenPipeError traceback when the reader closes early.
 signal.signal(signal.SIGPIPE, signal.SIG_DFL)
 
-DB = '/home/fabi/RE - Project Sylpheed/xenia-rs/sylpheed.db'
-c = duckdb.connect(DB, read_only=True)
-H = lambda x: '0x%08x' % x
+REGEN = "sylph-xexdb dis <xex|iso> --db sylpheed.db --analyze sql"
 
-REGEN = ("xenia-rs dis <xex|iso> --db sylpheed.db --analyze sql")
+
+def _resolve_db():
+    """Locate the database: `$SYLPH_XEXDB`, else `<repo root>/sylpheed.db`.
+
+    🔴 This used to be one hardcoded absolute path, and it pointed *inside*
+    `xenia-rs` -- a repository `docs/agents/CONSOLIDATION.md` archives in Phase 5
+    and drops in Phase 7. It resolved on exactly one machine and would have
+    started failing there too, with `duckdb` raising about a missing file rather
+    than anything saying why.
+
+    The database is a build artefact of several hundred MB and is not in the
+    repository, so there is nothing to fall back *to*: if neither location has
+    one, say so and print how to build it. A default that silently resolves to
+    the wrong database is worse than no default -- see issue #16.
+    """
+    env = os.environ.get('SYLPH_XEXDB')
+    if env:
+        p = pathlib.Path(env).expanduser()
+        if not p.is_file():
+            sys.exit(f'$SYLPH_XEXDB is set but is not a file:\n  {p}')
+        return p
+    # Relative to this script, not to the caller's cwd: `zq.py` is run from
+    # wherever the investigation happens to be.
+    p = pathlib.Path(__file__).resolve().parent.parent / 'sylpheed.db'
+    if p.is_file():
+        return p
+    sys.exit(
+        f'no database found.\n'
+        f'  looked for: {p}\n'
+        f'  set $SYLPH_XEXDB to an existing one, or build it with:\n'
+        f'    {REGEN}'
+    )
+
+
+# Resolved lazily enough that `zq.py` with no command still prints its usage on
+# a machine that has no database yet.
+_WANTS_DB = len(sys.argv) > 1 and sys.argv[1] not in ('-h', '--help', 'help')
+DB = _resolve_db() if _WANTS_DB else None
+c = duckdb.connect(str(DB), read_only=True) if DB is not None else None
+H = lambda x: '0x%08x' % x
 
 
 def _need(*tables):