feat: support personal skills in chats (#25366)

> Mux updated this PR on behalf of Mike.

## Stack Context

This PR builds on #25365 in the experimental personal skills stack and
completes the chat integration.

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?

Updates chattool skill formatting and `read_skill` resolution so tools
can read personal skills from the database, then injects personal skill
metadata into chatd prompts and registers the skill-reading tools when
skills are available.

This branch has also been merged with current `origin/main` to resolve
merge conflicts.

## Why?

The chattool and chatd changes need to land together so the intermediate
stack state stays buildable. This completes personal skill availability
in chats without syncing personal skills into workspace filesystems.

## Validation

- `go test -count=1 ./coderd/x/chatd/chattool -run
'TestFormatResolvedSkillIndex|TestReadSkillTool|TestReadSkillFileTool'`
- `go test -count=1 ./coderd/x/chatd -run
'TestPersonalSkillsInSystemPrompt|TestPersonalAndWorkspaceSkillCollisionInSystemPrompt|TestSkillIndexRefreshReplacesStaleAliases|TestFetchPersonalSkillMetadata|TestLoadPersonalSkillBody'`
- `go test -count=1 ./coderd -run 'Test.*UserSkill'`
- `git diff --cached --check`
- `make lint`
- pre-commit hook

coderd/userskills_test.go

@@ -667,8 +667,7 @@ func userSkillMarkdown(name string, description string, body string) string {
 func requireSDKErrorStatus(t *testing.T, err error, status int, msgAndArgs ...any) *codersdk.Error {
 	t.Helper()
 	require.Error(t, err, msgAndArgs...)
-	var sdkErr *codersdk.Error
-	require.ErrorAs(t, err, &sdkErr, msgAndArgs...)
+	sdkErr := coderdtest.SDKError(t, err)
 	require.Equal(t, status, sdkErr.StatusCode(), msgAndArgs...)
 	return sdkErr
 }