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

coderd/x/chatd/chattool/skill.go

@@ -15,7 +15,7 @@ import (
 )
 
 const (
-	maxSkillMetaBytes = 64 * 1024
+	maxSkillMetaBytes = workspacesdk.MaxSkillMetaBytes
 	maxSkillFileBytes = 512 * 1024
 )