refactor(web): drop the unconsumed observation surface

WebService exposed an observation surface nothing in production observes:
the web/providers-change event (declared, emitted on every provider
registration/disposal, rollback-yield ordered before the emit solely so a
throwing change listener unwinds the registration) and the aggregated
searchStatus()/fetchStatus() query with its WebCapabilityStatus union.
dsh-tool-web executes through ctx.web.search()/fetch() and routes on the
structured WebError codes selection throws at execution time; tool
registration follows product enablement, not provider availability. The
only listeners/callers were the web packages' own tests, and the
tool-web README / architecture.md prose claiming the tool 'reads only the
aggregated searchStatus()/fetchStatus()' had drifted from the call sites.

Remove the event declaration, both emits, and the rollback-before-emit
machinery (the plain ctx.effect disposer keeps HMR cleanup, matching
LlmService.registerAdapter). Remove searchStatus()/fetchStatus(),
resolveStatus(), and WebCapabilityStatus; the provider-private status()
stays as the execution-time selection input. Delete the listener-throw
rollback test, and rewrite every event/status assertion across the web
packages' tests onto caller-observable behavior: a successful
search()/fetch() or the structured WEB_PROVIDER_* codes. Regenerate the
cordis catalog; update the web/tool-web READMEs, the architecture.md web
paragraph, core-data-structures/web.md, and the type-equiv manifest; amend
the web capability seam RFC's facts to the shipped surface. This follows
the llm/adapter-change precedent: a boot-time backend-registry signal and
an availability probe distinct from executing both sit on the cut side of
its keep/cut criterion.

RFC: docs/rfc/implemented/simplification/2026-07-04-drop-unconsumed-web-observation-surface.md

packages/web/web-search-exa/tests/exa.spec.ts

@@ -205,12 +205,14 @@ describe('ExaSearchProvider error handling', () => {
 
 describe('web-search-exa plugin registration', () => {
   it('registers the provider into ctx.web (HMR-safe)', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () => jsonResponse({ results: [] })))
     const ctx = new Context()
     await ctx.plugin(WebService, { searchProvider: EXA_PROVIDER_ID })
     const fiber = await ctx.plugin(exaPlugin, { apiKey: 'exa-key' })
-    expect(ctx.web.searchStatus()).toEqual({ available: true, providerId: EXA_PROVIDER_ID })
+    await expect(ctx.web.search({ query: 'q' })).resolves.toMatchObject({ providerId: EXA_PROVIDER_ID })
     await fiber.dispose()
-    expect(ctx.web.searchStatus()).toEqual({ available: false, reason: 'configured-missing' })
+    await expect(ctx.web.search({ query: 'q' }))
+      .rejects.toThrow(expect.objectContaining({ code: 'WEB_PROVIDER_CONFIGURED_MISSING' }))
   })
 
   it('has no default export (namespace plugin export shape)', () => {
@@ -238,7 +240,6 @@ describe('web-search-exa plugin registration', () => {
       const ctx = new Context()
       await ctx.plugin(WebService, { searchProvider: EXA_PROVIDER_ID })
       const fiber = await ctx.plugin(exaPlugin, {})
-      expect(ctx.web.searchStatus()).toEqual({ available: true, providerId: EXA_PROVIDER_ID })
       await ctx.web.search({ query: 'q' })
       const [url] = fetchMock.mock.calls[0] as unknown as [string]
       expect(url).toBe('https://api.exa.ai/search')
@@ -256,7 +257,8 @@ describe('web-search-exa plugin registration', () => {
       const ctx = new Context()
       await ctx.plugin(WebService, { searchProvider: EXA_PROVIDER_ID })
       await ctx.plugin(exaPlugin, {})
-      expect(ctx.web.searchStatus()).toEqual({ available: false, reason: 'configured-unavailable' })
+      await expect(ctx.web.search({ query: 'q' }))
+        .rejects.toThrow(expect.objectContaining({ code: 'WEB_PROVIDER_CONFIGURED_UNAVAILABLE' }))
     } finally {
       if (prev !== undefined) process.env.EXA_API_KEY = prev
     }