refactor(foundation): hygiene pass across early-init, user-constants, system-defaults, chrono-tools

Six small fixes the 2026-05-15 module-by-module re-review surfaced:

- Consolidate `user-home-dir` -- canonical defconst stays in
  early-init.el (package-archive bootstrap needs it before normal
  modules load); user-constants.el switches to a `defvar` with the
  identical `(getenv "HOME")` expression so the module still loads /
  byte-compiles standalone, but at runtime early-init's defconst
  wins.
- Drop the redundant `(autoload 'env-bsd-p ...)` line in
  system-defaults.el.  The `(eval-when-compile (require
  'host-environment))` already exposes the symbol to the byte
  compiler, and at runtime host-environment is loaded earlier in
  init.el.  Added a comment documenting the boundary.
- Convert `cj/debug-modules` and `cj/use-online-repos` from `defvar`
  to `defcustom`, with `:type`, `:group 'cj`, and a top-level
  `(defgroup cj ...)` so both show up in M-x customize.
- Name the package-archive priorities in early-init.el.  Nine new
  defconsts replace the magic numbers (200 / 125 / 120 / 115 / 100 /
  25 / 20 / 15 / 5) with one constant each, plus a header comment
  explaining the local-first ordering and the gnu > nongnu > melpa >
  melpa-stable trust ranking within each tier.
- Delete the 19-line commented-out `use-package time` world-clock
  block in chrono-tools.el.  `time-zones` immediately above is the
  active replacement; git history preserves the old config if anyone
  needs it.
- Add coverage for `cj/tmr-select-sound-file`.  Collapsed the
  prefix-arg branch into a delegation to
  `cj/tmr-reset-sound-to-default` (single reset source) and
  extracted `cj/tmr--available-sound-files` as a pure helper that
  tests directly.  9 ERT tests across Normal / Boundary / Error
  cover the available-sounds helper, the reset path, the prefix-arg
  delegation (no prompt), the normal selection path, and the
  empty-dir / missing-dir / cancel boundaries.

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