config: --init-config CLI redesign + documentation rewrite (CLI half of #13)

[kay-ws]

Summary

CLI redesign + documentation rewrite — the second half of the two-PR
split agreed in #13. Pairs with #14.

--init-config previously decided between /etc/bcon/ and ~/.config/bcon/
via dirs::config_dir() of the calling user. Under sudo, this silently
writes to /root/.config/bcon/config.toml rather than /etc/bcon/... —
the production trap described in #13 (sec. 2 "init-config silently routes
to the caller's home"). This PR replaces the implicit destination with an
explicit target token, and rewrites docs/configuration.{md,ja.md} to
cover the layered model and the new CLI surface.

CLI: target token

The first comma-separated argument of --init-config=... is interpreted
as a WriteTarget when it is user, system, or starts with / or ~/.
Otherwise the entire list is treated as preset names — preserving every
pre-existing destination (legacy --init-config=vim,jp still writes to
~/.config/bcon/config.toml).

Form	Destination
bcon --init-config=system	/etc/bcon/config.toml
bcon --init-config=user	~/.config/bcon/config.toml
bcon --init-config=/tmp/x.toml	/tmp/x.toml (load via BCON_CONFIG)
bcon --init-config=~/foo.toml	$HOME/foo.toml (tilde-expanded; load via BCON_CONFIG)

sudo bcon --init-config=system now writes to /etc/bcon/config.toml
deterministically, regardless of the caller's $HOME.

Library API

• pub enum WriteTarget { User, System, Path(PathBuf) }
• pub fn parse_init_config_arg(arg: &str) -> (WriteTarget, Vec<String>)
• Config::write_config_with_preset(target: &WriteTarget, presets: &[&str]) —
  target no longer threaded through the preset list
• Config::path_for_target(&WriteTarget) (renamed from
  get_config_path_for_preset)
• private expand_tilde helper

WriteTarget and parse_init_config_arg are pub so distributors using
Config::default_template() (introduced in #14) can pick a destination
without re-implementing the CLI parser.

Documentation

docs/configuration.{md,ja.md} rewritten end-to-end to cover:

• the layered XDG model from config: layered XDG-style config discovery (engine + distributors API) #14 (built-in defaults → /etc/bcon/... →
  ~/.config/bcon/..., with helix/mpv-style merge semantics)
• the --init-config target-token syntax, presented as a Destinations
  table (4 forms) and a separate Examples subsection (typical
  invocations that complete with the standard layered load)
• the BCON_CONFIG env-var bypass (introduced in config: layered XDG-style config discovery (engine + distributors API) #14), documented with
  two use cases: loading an ad-hoc config generated by a path-target
  --init-config, and BCON_CONFIG=/dev/null bcon for clean-defaults
  A/B-comparison
• a heading rename (Example Config → Configuration File Example) so
  the upstream-content section name no longer collides with the new
  ### Examples subsection

CLAUDE.md's Configuration section gains a short pointer to the new
behavior.

Wording in src/config/mod.rs (introduced via #14) is forward-compat'd:
the load_layered_from_paths doc comment ("up to three layers") and the
test name (test_load_three_layer_merge) are renamed to "multi-layer"
versions so future overlays (per-host, per-project, etc.) don't make the
prose stale.

Backward compatibility

• --init-config (no argument): user destination, unchanged.
• Preset-only forms (e.g. --init-config=vim,jp): the detection rule
  short-circuits to WriteTarget::User when the first token is not a
  recognized target, so the destination stays at
  ~/.config/bcon/config.toml.
• --force semantics unchanged.
• One public-API rename (get_config_path_for_preset → path_for_target)
  and one signature change (write_config_with_preset takes
  &WriteTarget).

Test plan

cargo test --bin bcon config::tests: 11/11 passing (9 inherited
from #14, 2 new).

New test	Covers
test_parse_init_config_arg_cases	target detection over all CLI shapes (user, system, absolute path, ~/)
test_expand_tilde_basics	tilde expansion edge cases

Full cargo test: 53 passed; 0 failed; 1 ignored (seatd,
env-dependent). cargo build --release: clean, no new warnings.

Manual verification

End-to-end verified on a separate Arch VM (PR2 binary deployed alongside
the AUR-installed v1.3.4 as /usr/local/bin/bcon-pr2, original config
files preserved by mv-aside):

• sudo bcon --init-config=system writes to /etc/bcon/config.toml with
  root:root ownership; no /root/.config/bcon/config.toml is created
  — the production trap from Proposal: Layered XDG-style config discovery (/etc/ + ~/.config/ merge) #13 (sec. 2) is eliminated.
• bcon --init-config=user,vim,jp writes to
  $XDG_CONFIG_HOME/bcon/config.toml.
• bcon --init-config=/tmp/x.toml,vim writes to the requested absolute
  path.
• bcon --init-config=~/foo.toml,vim tilde-expands inside the binary
  (the shell does not expand ~ in =... context, so the literal
  ~/foo.toml reaches parse_init_config_arg and expand_tilde resolves
  it to /home/<user>/foo.toml).

Refs

Refs #13 (CLI half). Pairs with #14.

[sanohiro]

Thanks — the root-shadow trap is properly eliminated now. Docs rewrite is thorough too. This closes the #13 proposal cleanly.