Rename rfmt to kenshin

[sorafujitani]

Renames the project rfmt → kenshin, per the plan in #110 ("Rename: rfmt → kenshin" section). The name is verified free on RubyGems, and sorafujitani/kenshin is unused on GitHub.

What changes

• Gem kenshin (version stays 1.7.0; the bump happens at release). Executables kenshin / kenshin-lsp.
• Ruby module Kenshin, with Rfmt = Kenshin kept as a deprecation alias for one minor release.
• Rust extension ext/kenshin (crate/lib name drives the magnus init symbol; require 'kenshin' verified after rake compile).
• Config discovery prefers the .kenshin.yml family and still accepts the legacy .rfmt.yml names afterward, in both the Rust resolver and the Ruby finders (tested on both sides; transition window noted in code).
• Cache dir ~/.cache/kenshin. Env vars KENSHIN_DEBUG/LOG/VERBOSE (old names dropped; flagged in the changelog).
• Docs, editor guides, README badges, CI (the install-check job now exercises the kenshin exe), CHANGELOG entry describing the rename and migration path.

What deliberately keeps the old name

• Repo URLs stay sorafujitani/rfmt until the GitHub repo itself is renamed (that is the maintainer's post-merge step; redirects then cover stragglers). The gemspec metadata already points to sorafujitani/kenshin.
• The parity fixture generator loads the deleted PrismBridge from git history, so its internal lib/rfmt/... path and Rfmt:: reference are historical by nature (verified idempotent: fixtures regenerate byte-identically).
• Dated records (docs/priv, historical benchmark JSON) are untouched.
• A full grep -rniE "\brfmt\b" inventory over lib/ext/exe/spec/scripts/.github was reviewed hit-by-hit; every remaining hit falls in the categories above.

Verification

• rspec 168/0 (two new config-compat examples); corpus check 46/46; parity fixtures idempotent; cargo test 147 / clippy --all-targets -D warnings / fmt clean; rubocop clean
• gem build kenshin.gemspec: name/executables/extensions correct; ruby_lsp_addon metadata intact
• Real CLI: bundle exec kenshin --write honors .kenshin.yml (indent 4) and legacy .rfmt.yml (indent 3) in separate temp projects; --check exit codes; kenshin version reports the new name
• Both Kenshin.format and legacy Rfmt.format return identical output

After merge (maintainer steps)

1. Rename the GitHub repo to sorafujitani/kenshin (old URLs redirect)
2. Follow-up PR: flip the remaining in-code repo URLs to the new home
3. Release sequencing per codename/v2: performance and stability improvements (move parsing to Rust via ruby-prism) #110: first kenshin release carries the native-parsing migration + rename changelog; a final rfmt gem release points users here (post_install_message)

[sorafujitani]

Decision: keeping the name rfmt (codename v2). Closing unmerged.