feat(pkg,nix): migrate build orchestrators + debuggers to nix; drop toolchains

Migrated to Home-Manager (nix/common.nix):

  cmake, ninja, ccache, sccache (build orchestrators — they only
    delegate to whatever compiler is on PATH, don't ship one
    themselves, so no ABI shadowing risk)
  valgrind (instruments at syscall/library boundary; works fine with
    pacman-built target binaries)
  doxygen (source-only documentation tool; was accidentally dropped
    in the previous sweep — restored here)

Dropped entirely (per-project flake.nix + direnv .envrc instead):

  clang, lld, mold, rustup, npm

  Rationale: these are language/toolchain-specific compilers and
  linkers; when a project needs them, the project's own flake provides
  the version it wants, pinned in flake.lock. base-devel still ships
  gcc/ld/as/make for general-purpose system builds and one-offs.

  npm is also dropped from pacman; nodejs_24 in nix/common.nix already
  ships npm for the editor/AI-agent path. Project-side npm comes via
  per-project flake when needed.

Updated nix/common.nix policy comment to match: only ban the actual
compilers/linkers (cc/gcc/clang/ld) and forbidden runtimes (cargo,
rustc, go, python3) — build orchestrators and instrumentation tools
are explicitly allowed.

The --- dev --- section in base.txt is now just perf and podman-*
(kernel-coupled / system-runtime-coupled).

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