docs: drop the plans directory

The specs describe shipped features — incremental builds, the live
view, overlayfs staging, pkh new, pkh test, pkh lint phase 1 — and
the satellite-tools plan is implemented in full (no shell-outs to
dpkg-architecture, dpkg-checkbuilddeps or dpkg-parsechangelog
remain). The two unshipped proposals (the lint porting track and the
data/configuration architecture) stay in whatever notes the project
keeps outside the tree; the README remains the user-facing
documentation.

src/lint/mod.rs

@@ -1,6 +1,6 @@
 //! `pkh lint`: lint a Debian source tree, with lintian feature parity.
 //!
-//! The strategy is *wrap first, port second* (see `plans/pkh-lint.md`): the
+//! The strategy is *wrap first, port second*: the
 //! [wrapper](wrapper) runs the installed lintian over an ephemeral source
 //! package for day-one parity with every lintian check, while the native
 //! engine (check registry, collectors, emitter) hosts pkh-specific workflow

src/new/mod.rs

@@ -1,5 +1,4 @@
-//! `pkh new`: interactive-first package scaffolding (see
-//! `plans/pkh-new.md`).
+//! `pkh new`: interactive-first package scaffolding.
 //!
 //! This module orchestrates a scaffold run: target directory checks, project
 //! detection, in-memory rendering of every file (all-or-nothing write), the