fix(init): stop --force wiping config.toml; guard skill overwrites

[markovejnovic]

Summary

hm init --force used to remove_dir_all the entire .hm/ directory before
rewriting a single template. In a repo with multiple pipelines (e.g.
pipeline.py + deploy.py, as examples/elixir-phoenix ships) plus a cloud
config.toml, --force silently destroyed everything but the one regenerated
template — silent data loss. This PR makes --force overwrite only the
target template file (std::fs::write clobbers just that path; .hm/ already
exists, so create_dir_all is a no-op), so config.toml, .gitignore, and
co-resident pipelines survive.

It also hardens write_skills, which previously clobbered each SKILL.md
unconditionally — destroying local edits on any re-run of hm init. It now
takes a force flag (threaded from args.force):

• absent file → installed,
• present but unchanged from the bundled copy → silent no-op,
• present with local edits → skipped with a tracing::warn! pointing at --force.

The info log now distinguishes "installed" (new) from "overwrote" (force) for
honest feedback.

Findings addressed

• init-force-wipes-dir-1 — --force no longer wipes the whole .hm/ dir.
• init-skills-overwrite-1 — write_skills no longer silently clobbers customized skills.

Verification

Worktree /tmp/hmfix/9, branch rebased onto current origin/main so the PR
contains only the fix commit (3 files: init.rs, cmd_init.rs, CHANGELOG.md).

• cargo test -p harmont-cli --lib commands::init → 4 passed, 0 failed
  (new unit tests: install-when-absent, preserve-without-force,
  force-overwrites, unchanged-no-op).
• cargo test -p harmont-cli --test cmd_init init_force_preserves_coresident_files → 1 passed
  (asserts config.toml + deploy.py survive while the template is rewritten).
• cargo test -p harmont-cli --test cmd_init → 23 passed, 1 failed. The
  single failure, init_ts_templates_roundtrip_render, is a pre-existing,
  environment-level failure (Bun cannot resolve the pipeline export in the
  bundled @harmont/hm/index.mjs). It reproduces identically with origin/main's
  copy of the touched files and is untouched by this change — not a regression.
• cargo clippy -p harmont-cli --lib → clean, no warnings.

Judge verdict

Approved. Blocking issues: none.

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Generated by the Harmont release-readiness workflow (automated; needs human review before merge). 🤖