Allow installing agent skills into .agents/skills

[janniklasrose]

Changes

Allow installing agent skills into vendor-agnostic .agents/skills directory

Why

Emerging standard: https://agentskills.io/home#where-can-i-use-agent-skills

Tests

Added tests; verified e2e in local

[eng-dev-ecosystem-bot]

Integration test report

Commit: 8180cad

Run: 31164651453

	Env	💚​RECOVERED	🙈​SKIP	✅​pass	🙈​skip	Time
💚​	aws linux	4	4	291	1116	4:32
💚​	aws windows	4	4	293	1114	3:28
💚​	azure linux	4	4	290	1116	5:05
💚​	azure windows	4	4	292	1114	4:58
💚​	gcp linux	1	5	291	1116	4:50
💚​	gcp windows	1	5	293	1114	3:44

8 interesting tests: 4 RECOVERED, 4 SKIP

	Test Name	aws linux	aws windows	azure linux	azure windows	gcp linux	gcp windows
💚​	TestAccept	💚​R	💚​R	💚​R	💚​R	💚​R	💚​R
🙈​	TestAccept/bundle/invariant/no_drift	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/bundle/resources/vector_search_endpoints/drift/recreated_same_name	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/bundle/resources/vector_search_indexes/recreate/embedding_dimension	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
🙈​	TestAccept/ssh/connection	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S	🙈​S
💚​	TestFetchRepositoryInfoAPI_FromRepo	💚​R	💚​R	💚​R	💚​R	🙈​S	🙈​S
💚​	TestFetchRepositoryInfoAPI_FromRepo/root	💚​R	💚​R	💚​R	💚​R
💚​	TestFetchRepositoryInfoAPI_FromRepo/subdir	💚​R	💚​R	💚​R	💚​R

Top 3 slowest tests (at least 2 minutes):

duration	env	testname
3:20	aws windows	TestAccept
3:07	azure windows	TestAccept
3:00	gcp windows	TestAccept