bug: codegraph watch does not accept --db, unlike other commands

[carlos-alm]

Found during dogfooding v3.9.4

Severity: Medium
Command: codegraph watch

Every other command that operates on the graph DB accepts --db <path> to point at a DB outside the current working directory (build, stats, query, fn-impact, impact, etc.). watch does not — it rejects the flag with error: unknown option '--db' and only supports the positional [dir].

Reproduction

npx codegraph watch --db /abs/path/to/graph.db
# error: unknown option '--db'

Expected behavior

watch --db <path> <dir> should start the watcher against <dir> and write incremental updates to <path>, matching the behavior of build --db.

Actual behavior

Flag is rejected.

Impact

Users running watch mode from one directory against a graph DB in another (common in monorepo setups and MCP multi-repo workflows) must cd to the repo root, which defeats the purpose of having the --db flag in the other commands.

Suggested fix

Wire --db <path> into the watch command in src/cli.ts the same way it's wired in build, and thread it down to the watcher's DB handle.