feat: add glab search semantic command

Summary

• Adds glab search semantic command to query the GitLab semantic code search REST endpoint (GET /projects/278964/search/semantic — added in gitlab!227817 (merged))
• Supports natural language queries with optional directory path filtering, result count limiting, and JSON output
• Project resolves from git remote by default; override with -R/--repo

Closes https://gitlab.com/gitlab-org/cli/-/work_items/8227

MR chain

Repo	MR	Status	Description
gitlab-org/gitlab	gitlab!227816 (merged)	✅ merged	Extract post-processing concern
gitlab-org/gitlab	gitlab!227817 (merged)	✅ merged	Add REST endpoint
gitlab-org/gitlab	gitlab!229665 (merged)	✅ merged	Add gPAT support (read_code permission)
gitlab-org/gitlab	gitlab!228569 (merged)	🔄 open	FF mcp_semantic_code_search_use_rest_api
gitlab-org/gitlab	gitlab!227818	🔄 open	Sunset SemanticCodeSearchService
gitlab-org/cli	!3084	🔄 open	Add glab search semantic CLI command
gitlab-org/editor-extensions/gitlab-lsp	gitlab-org/editor-extensions/gitlab-lsp!3157	🔄 open	Add gitlab_semantic_code_search to tool catalog
ai-assist	gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!5174	🔄 open	Add SemanticCodeSearch tool implementation

Epic: https://gitlab.com/groups/gitlab-org/-/epics/21285

Flags

Flag	Short	Default	Description
--query	-q	required	Natural language search query
--directory-path	-d	—	Restrict search to files under this path
--knn	—	server default (64)	Nearest neighbours to retrieve
--limit	-l	server default (20)	Maximum results
--output	-F	text	Output format: text or json

How to test

Unit tests:

go test ./internal/commands/search/...

Local GDK verification:

# Unprovisioned project → 422 (not 500)
curl -sk -o /dev/null -w "%{http_code}\n" --header "PRIVATE-TOKEN: $TOKEN" \
  "https://gdk.test:3443/api/v4/projects/7/search/semantic?q=auth"

# Happy path → 200 with results
glab search semantic -q "authentication" -R "gitlab-duo/test"