feat: add personal skill resolver (#25362)

> Mux updated this PR on behalf of Mike.

## Stack Context

This stack splits experimental personal skills into smaller reviewable
PRs. Personal skills are user-owned `SKILL.md` files stored by Coder and
injected into chatd alongside workspace skills.

Stack order:
1. #25362 personal skill resolver
2. #25363 storage, permissions, API, and SDK
3. #25365 API test coverage
4. #25366 chattool and chatd integration
5. #25066 settings UI and docs
6. #25386 personal skills slash menu

## What?

Adds the shared personal skill parser and resolver package, plus
reusable skill-name validation exported from `workspacesdk`.

The parser enforces the full personal skill contract: max raw size,
kebab-case name, max name length, and non-empty body.

## Why?

The rest of the stack needs one source-aware resolver for personal and
workspace skills, including collision handling and qualified aliases.
Keeping personal skill constraints in the parser prevents callers from
accidentally parsing invalid personal skills.

## Validation

- `go test ./coderd/x/skills ./codersdk/workspacesdk`
- pre-commit hooks on this branch

codersdk/workspacesdk/frontmatter.go

@@ -7,10 +7,23 @@ import (
 	"golang.org/x/xerrors"
 )
 
+// SkillNameRegex is the regular expression used to validate kebab-case skill names.
+const SkillNameRegex = "^[a-z0-9]+(-[a-z0-9]+)*$"
+
+// MaxSkillMetaBytes is the maximum raw Markdown size accepted for a skill meta file.
+const MaxSkillMetaBytes = 64 * 1024
+
+// SkillNamePattern is the compiled pattern used to validate kebab-case skill names.
+var SkillNamePattern = regexp.MustCompile(SkillNameRegex)
+
 // markdownCommentRe strips HTML comments from skill file bodies so
 // they don't leak into the LLM prompt.
 var markdownCommentRe = regexp.MustCompile(`<!--[\s\S]*?-->`)
 
+// ErrFrontmatterNameRequired is returned by ParseSkillFrontmatter when
+// the frontmatter is missing a required name field.
+var ErrFrontmatterNameRequired = xerrors.New("frontmatter missing required 'name' field")
+
 // ParseSkillFrontmatter extracts name, description, and the
 // remaining body from a skill meta file. The expected format is
 // YAML-ish frontmatter delimited by "---" lines:
@@ -65,9 +78,7 @@ func ParseSkillFrontmatter(content string) (name, description, body string, err
 	}
 
 	if name == "" {
-		return "", "", "", xerrors.New(
-			"frontmatter missing required 'name' field",
-		)
+		return "", "", "", xerrors.Errorf("%w", ErrFrontmatterNameRequired)
 	}
 
 	// Everything after the closing delimiter is the body.

codersdk/workspacesdk/frontmatter_test.go

@@ -117,6 +117,7 @@ func TestParseSkillFrontmatter(t *testing.T) {
 		_, _, _, err := workspacesdk.ParseSkillFrontmatter(
 			"---\ndescription: no name\n---\n",
 		)
+		require.ErrorIs(t, err, workspacesdk.ErrFrontmatterNameRequired)
 		require.ErrorContains(t, err, "frontmatter missing required 'name' field")
 	})