`worklog init`: starter config writer

[Saber5656]

Title

worklog init: starter config writer

Summary

Implement worklog init: write the commented starter config
(config.Starter(), issue 05) to the default (or --config) path with
safe-write semantics, refusing to overwrite without --force.

Context

This is the second and last legitimate write path (invariant I3). It reuses
the atomic-write mechanics but targets the config location and must create
~/.config/worklog/ correctly on first run.

Scope

• internal/cli/init.go (replace stub) + tests.

Detailed Requirements

1. Target: --config value if given, else config.DefaultPath().
2. Behavior: existing file + no --force → message
   config already exists: <path> (use --force to overwrite) to stderr,
   exit 1. Otherwise: create parent dirs 0700, atomic temp+rename write,
   file mode 0600 (config may later hold llm.api_key), then print
   wrote <path> to stderr and, to stdout, the 3-line quickstart:
   Edit the config, then run: / worklog doctor / worklog daily.
3. Symlink final component → refuse (same rule as 22; reuse
   output-package internals via a small exported helper
   output.WriteFileSafe(path string, content []byte, force bool) error
   — refactor 22's core into this helper in THIS issue, keeping 22's tests
   green).
4. The written file must round-trip: config.Load(path) → zero warnings,
   zero validation errors. This holds because config.Starter() (issue 05)
   ships [llm] enabled = false with the guidance comment — identical to
   the built-in default (DESIGN §5), so no starter-vs-default divergence
   exists.
5. init ignores --date/--lang etc. (not registered on its FlagSet, 07
   already scoped flags).

Acceptance Criteria

• Fresh HOME: creates dirs+file with exact Starter() bytes, 0600/0700
  modes, correct stdout/stderr split, exit 0.
• Existing config without --force → exit 1, file untouched
  (hash-asserted); with --force → replaced atomically.
• Symlinked config path → refused, exit 1.
• Round-trip: Load on the written file → no warnings, no validation
  errors.
• XDG_CONFIG_HOME respected when set (test both with and without).
• 22's test suite still green after the WriteFileSafe refactor.

Validation

go test -race -cover ./internal/cli/ ./internal/output/ ./internal/config/
in PR (three packages touched).

Dependencies

05, 07, 22 (hard dependency: this issue refactors 22's write core into the
shared output.WriteFileSafe helper).

Non-goals

Interactive prompts/wizards, config migration between versions (post-v1),
editing existing configs.

Design References

• docs/DESIGN.md §5, §10, §1.2 (I3)

---

Source of truth: docs/issues/27-cmd-init.md (PR #1, branch docs/v1-design). If this issue and the repo docs disagree, the docs win. Execution order and dependencies: docs/ISSUE_PLAN.md (this is issue 27 of 33).