docs: adopt Conventional Commits for commit messages

Vendor the v1.0.0 specification as docs/conventional-commits.md and record
the convention in CLAUDE.md: type and optional scope, where the scope names
the module or host, and no Gitea-specific references, since this repository
is mirrored to GitHub where issue and pull-request numbers resolve to
unrelated things.

CLAUDE.md

@@ -18,6 +18,9 @@ The domain model (Host, Module, Skeleton, Auto-loader, Enable convention, overla
 - Comments posted to Gitea (pull requests, issues, reviews) go out under the operator's account, so sign every one to make clear the author is the agent, not the operator.
   End the comment with a `— Claude` sign-off.
   (A dedicated bot account may replace this later; until then, the sign-off is the only marker.)
+- Commit messages follow Conventional Commits, specified in `docs/conventional-commits.md`.
+  Scope is the module or host the change belongs to (`fish`, `nvim`, `neogaia`), omitted for repo-wide changes.
+  Keep messages free of Gitea-specific references: this repository is mirrored to GitHub, where issue and pull-request numbers resolve to unrelated things.
 
 ## Gotchas
 

docs/conventional-commits.md

@@ -0,0 +1,57 @@
+# Conventional Commits
+
+> Sourced from [conventionalcommits.org/en/v1.0.0](https://www.conventionalcommits.org/en/v1.0.0/)
+> by the Conventional Commits authors, licensed under
+> [CC BY 3.0](https://creativecommons.org/licenses/by/3.0/).
+
+---
+
+## Summary
+
+The Conventional Commits specification is a lightweight convention on top of commit messages.
+It provides an easy set of rules for creating an explicit commit history; which makes it easier to write automated tools on top of.
+This convention dovetails with [SemVer](http://semver.org), by describing the features, fixes, and breaking changes made in commit messages.
+
+The commit message should be structured as follows:
+
+```
+<type>[optional scope]: <description>
+
+[optional body]
+
+[optional footer(s)]
+```
+
+The commit contains the following structural elements, to communicate intent to the consumers of your library:
+
+1. **fix:** a commit of the _type_ `fix` patches a bug in your codebase (this correlates with `PATCH` in Semantic Versioning).
+2. **feat:** a commit of the _type_ `feat` introduces a new feature to the codebase (this correlates with `MINOR` in Semantic Versioning).
+3. **BREAKING CHANGE:** a commit that has a footer with a token `BREAKING CHANGE:`, or appends a `!` after the type/scope, introduces a breaking API change (correlating with `MAJOR` in Semantic Versioning). A BREAKING CHANGE can be part of commits of any _type_.
+4. _types_ other than `fix:` and `feat:` are allowed, for example `build:`, `chore:`, `ci:`, `docs:`, `style:`, `refactor:`, `perf:`, `test:`, and others.
+5. _footers_ other than `BREAKING CHANGE: <description>` may be provided and follow a convention similar to [git trailer format](https://git-scm.com/docs/git-interpret-trailers).
+
+Additional types are not mandated by the Conventional Commits specification, and have no implicit effect in Semantic Versioning (unless they include a BREAKING CHANGE).
+A scope may be provided to a commit's type, to provide additional contextual information and is contained within parenthesis, e.g., `feat(parser): add ability to parse arrays`.
+
+---
+
+## Specification
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC 2119](https://www.ietf.org/rfc/rfc2119.txt).
+
+1. Commits MUST be prefixed with a type, which consists of a noun, `feat`, `fix`, etc., followed by the OPTIONAL scope, OPTIONAL `!`, and REQUIRED terminal colon and space.
+2. The type `feat` MUST be used when a commit adds a new feature to your application or library.
+3. The type `fix` MUST be used when a commit represents a bug fix for your application.
+4. A scope MAY be provided after a type. A scope MUST consist of a noun describing a section of the codebase surrounded by parenthesis, e.g., `fix(parser):`.
+5. A description MUST immediately follow the colon and space after the type/scope prefix. The description is a short summary of the code changes, e.g., _fix: array parsing issue when multiple spaces were contained in string_.
+6. A longer commit body MAY be provided after the short description, providing additional contextual information about the code changes. The body MUST begin one blank line after the description.
+7. A commit body is free-form and MAY consist of any number of newline separated paragraphs.
+8. One or more footers MAY be provided one blank line after the body. Each footer MUST consist of a word token, followed by either a `:<space>` or `<space>#` separator, followed by a string value (this is inspired by the [git trailer convention](https://git-scm.com/docs/git-interpret-trailers)).
+9. A footer's token MUST use `-` in place of whitespace characters, e.g., `Acked-by` (this helps differentiate the footer section from a multi-paragraph body). An exception is made for `BREAKING CHANGE`, which MAY also be used as a token.
+10. A footer's value MAY contain spaces and newlines, and parsing MUST terminate when the next valid footer token/separator pair is observed.
+11. Breaking changes MUST be indicated in the type/scope prefix of a commit, or as an entry in the footer section.
+12. If included as a footer, a breaking change MUST consist of the uppercase text `BREAKING CHANGE`, followed by a colon, space, and description, e.g., _BREAKING CHANGE: environment variables now take precedence over config files_.
+13. If included in the type/scope prefix, breaking changes MUST be indicated by a `!` immediately before the `:`. If `!` is used, `BREAKING CHANGE:` MAY be omitted from the footer section, and the commit description SHALL be used to describe the breaking change.
+14. Types other than `feat` and `fix` MAY be used in your commit messages, e.g., _docs: correct spelling of CHANGELOG_.
+15. The units of information that make up Conventional Commits MUST NOT be treated as case sensitive by implementors, with the exception of BREAKING CHANGE which MUST be uppercase.
+16. BREAKING-CHANGE MUST be synonymous with BREAKING CHANGE, when used as a token in a footer.