docs(plan): add dependency-surface inventory implementation plan

A task-by-task plan to make every dependency surface — across the
maintainer harness and the generated project — discoverable via an
in-repo, test-asserted surface-map plus `just deps`, with Renovate as
the freshness/cross-check layer and zizmor as Actions pin enforcement.
Purely additive: modifies existing `.jinja` only, with no new template
files and no migrations.

Author: maybebyte