Fix compilation warnings on modern Elixir

[gilbertwong96]

Summary

Resolves all compilation warnings emitted when building this project on modern Elixir/OTP. Verified with mix compile --warnings-as-errors on Elixir 1.20 / OTP 29.

Changes

• config/config.exs — Replace deprecated use Mix.Config with import Config (the Mix.Config macro has been deprecated since Elixir 1.11).
• lib/mix/tasks/pre_commit.ex — Replace module-body Application.get_env/2 calls with Application.compile_env/3, supplying the previous || default fallback as the third argument. Reading config at compile time via compile_env/3 is the recommended pattern and eliminates the "discouraged in the module body" warning.
• mix.exs — Bump required Elixir from ~> 1.2 to ~> 1.16. The previous constraint was older than the ~> 1.16 value already documented in the @moduledoc, and ~> 1.2 no longer resolves cleanly on current Elixir releases.
• mix.lock — Refresh transitive deps (ex_doc, earmark_parser, makeup, makeup_elixir, makeup_erlang, nimble_parsec) that mix deps pulled in for the build.

Verification

$ rm -rf _build/dev/lib/pre_commit
$ mix compile
Compiling 2 files (.ex)
Generated pre_commit app

$ mix compile --warnings-as-errors
# (no output, exits 0)

Notes

• The only remaining warnings on a clean mix compile come from inside deps (makeup, ex_doc) and are out of scope for this change.
• Behaviour of the pre_commit Mix task and the git pre-commit hook is unchanged — this is a refactor / build hygiene PR.

Test plan

• mix compile runs clean with no project warnings
• mix compile --warnings-as-errors exits 0
• Manual: drop a pre_commit config into a host project, commit a change, confirm pre-commit task still runs commands and halts correctly