feat(pkg,nix): migrate 14 leaf tools to nix; drop 6 unused packages

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

  duf, gdu, nmap, procs, yazi, difftastic, direnv (was duplicated),
  git-absorb, samply, strace, t-rec, act, pandoc, gdb, lldb

  lldb stays nix-only (no longer in base.txt) — per user policy, only
  ever used to debug own builds, so glibc/kernel ABI skew vs the
  pacman-built system isn't a concern. Same logic could extend to
  valgrind, but valgrind has tighter glibc compat needs; perf links
  against kernel ABI and must match the running kernel. Both stay
  pacman.

  uv was already in nix; removed pacman duplicate.

Kept on pacman (cannot migrate without breaking system builds, per
nix/common.nix policy that bans compilers/linkers/build systems on
PATH):

  ccache, clang, cmake, lld, mold, ninja, npm, rustup, sccache,
  podman-compose, podman-docker (system runtime integration),
  perf, valgrind (kernel/glibc-coupled), unzip (transitive via base),
  doxygen (huge nixpkgs closure not worth it).

Dropped entirely (unused):

  android-tools, go, gpg-tui, luarocks (was for Mason-managed nvim
  plugin deps; Mason is gone), bash-completion (zsh-only setup),
  pandoc-bin (replaced by nix pandoc).

  jdk21-openjdk kept — still needed for the groovy/jenkins toolchain
  paths in nvim.

Rewrote the --- dev --- section comment to explain the policy.

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