Align skills REST API contract with RESTful conventions

[JAORMX]

Summary

The skills endpoints (merged as 501 stubs in #3801) used verb-based routes
(POST /install, POST /uninstall) that don't follow REST conventions. This
PR fixes the API contract before the service implementation lands, so #3804
can rebase onto clean routes.

• POST /skills/install → POST /skills/ — standard resource creation
• POST /skills/uninstall → DELETE /skills/{name} — idiomatic resource deletion via path param + ?scope= query param
• Removed uninstallSkillRequest body type (no longer needed)
• Added Enums(user, project) constraint to all three scope query params (list, info, uninstall) for machine-readable OpenAPI validation
• Documented Location header on 201 install response
• Added 400/404/409 failure codes to swagger annotations where appropriate
• Added ValidateScope() function for scope validation at API boundaries
• Added Scope field to InfoOptions so the info endpoint can accept scope filtering

Route comparison

Endpoint	Before	After
Install	POST /api/v1beta/skills/install	POST /api/v1beta/skills
Uninstall	POST /api/v1beta/skills/uninstall (JSON body)	DELETE /api/v1beta/skills/{name}?scope=
List	GET /api/v1beta/skills	unchanged (added ?scope= enum)
Info	GET /api/v1beta/skills/{name}	unchanged (added ?scope= enum + 404)

All handlers remain 501 stubs — zero service code is touched. #3804 will
rebase onto this and adapt its handlers to the new routes.

Why now?

Changing routes after the service implementation lands means rewriting handler
code, tests, and client assumptions simultaneously. Doing it while everything
is still a stub makes the change trivial and reviewable.

Refs: #3648, #3741

Test plan

• task lint — 0 issues
• task test — all passing
• task docs — swagger regenerated with enum constraints
• TestValidateScope — 8 cases: valid scopes, unknown values, case sensitivity, whitespace, trailing spaces, and error message content assertions

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.87%. Comparing base (02496a2) to head (cbd190f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/api/v1/skills.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3818      +/-   ##
==========================================
- Coverage   66.88%   66.87%   -0.01%     
==========================================
  Files         439      439              
  Lines       43437    43443       +6     
==========================================
+ Hits        29051    29053       +2     
- Misses      12134    12138       +4     
  Partials     2252     2252              

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[aponcedeleonch]

LGTM and aligned with the comments that Chris had. I'll leave it up to you to wait for Chris's review or go ahead and merge