Replace Specialist Agent with full Skill editing in the integration editor

[mattprintz]

Summary

Removes the "Specialist Agent" / AdhocAPI integration from the integration
editor and replaces it with full management of Agent Skills. Skills are now a
mutable integration provider: you can create, edit, and delete local skills,
add remote (URL-based) skills, edit their progressively-disclosed resources,
and import a skill from a SKILL.md file or a .zip. Skills bundled within a
context remain read-only (view mode).

Backend

• SkillIntegrationProvider is now a MutableBaseIntegrationProvider:
  add/update/remove_integration (local + remote) and
  add/update/remove_resource for local skills, with SKILL.md
  (frontmatter + body) serialization and file writes under reference/script/
  asset (singular and plural) and examples.
• Deterministic integration uuids (agent-skill:<corpus>:<slug>); context
  skills namespaced under context-<slug> so the UI renders them read-only.
• Local file-resource discovery scans on-disk resource dirs; body-reference
  extraction is used only for remote skills and normalizes ./ prefixes
  (fixes phantom/duplicate resources).
• add_integration(preview=True, ...) builds an unsaved skill from a URL or
  raw SKILL.md content so the editor can prefill before saving.
• Write-location resolution (first existing-writable, else first creatable)
  across the skill search roots; remote skills persisted as skills.json
  entries. Path-containment guards on all resource writes.
• New DELETE /beaker/integrations/{session}/{id} → remove_integration.
• System-preamble refresh is now lazy: an integration mutation marks it dirty
  and it rebuilds at the next agent turn — no session restart.

Frontend

• New SkillIntegrationEditor.vue (source type/URL, name, description, metadata
  incl. allowed-tools chips, instructions body) and editable
  SkillResourcePanel.vue (add/edit/delete file & example resources; shows
  pending-import resources).
• agent-skill uses the MCP-style viewer/editor split; Specialist Agent removed
  from the New-Integration menu and dispatch.
• Import: skillArchive.ts unzips in the browser (fflate), locates
  SKILL.md (root or one dir deep, fails on >1), enumerates valid resource
  files, skips non-text with a surfaced count. Upload button + panel-wide drop
  target.
• Delete button; Save is always present for editable skills (disabled until
  dirty). Left-panel Edit/View label now correct for skills.
• Layout fix: header and action bar stay pinned; only the content between them
  scrolls (also fixes the same issue for MCP).

Docs & tests

• Updated concept_integrations.md, integration_providers.md,
  integrations_api.md (incl. correcting pre-existing remove_resource /
  display_name / package-path inaccuracies).
• Backend tests/integrations/skills/test_skill_mutation.py (+ extraction
  tests); frontend src/util/__tests__/skillArchive.spec.ts.

Behavior notes

• Remote skills: source type + URL are the only editable fields; fetched content
  is read-only. Source type is fixed after creation (delete + recreate to
  change). Binary resource content is not yet supported (text-only).

Follow-ups (out of scope)

• Remove the adhoc backend provider + adhoc_api dependency (UI-only removal
  here). Existing adhoc integrations now render a blank editor — deprecated and
  believed unused.
• Hard backend read-only guard for context-bundled integrations (tracked
  separately); currently enforced frontend-only, matching MCP.
• Binary resource support (base64 content_encoding marker); remote name/slug
  override (needs skills.json object-entry parsing).

Testing

• Backend: hatch run pytest tests/ — full suite green (skills suite: 99).
• Frontend: vitest run green; vue-tsc --build 0 errors.