Remove dead vMCP discovery seam and default router

[tgrunnagle]

Summary

Why: Once Serve became the sole *Server constructor and always builds the domain core, the legacy server.New path (s.core == nil) became unreachable. Capability aggregation and call admission now run entirely through the core, so the discovery middleware, its context seam, the discovery.Manager, and the context-coupled defaultRouter were dead code reachable only on that path. This is the final stage of the staged dead-code removal under #5621 (after #5622 and #5625).

What:

• Delete the pkg/vmcp/discovery package (manager, middleware, context seam, mocks) — it had zero importers after the seam removal.
• Delete router.defaultRouter (+ test), whose only coupling was reading discovered capabilities from context; core.New only needs a non-nil router for workflow validation, which does not route, so the CLI now passes an empty NewSessionRouter(&vmcp.RoutingTable{}).
• Drop the discovery.Manager parameter from server.New (7 → 6 params) and remove the field, assignment, the s.core == nil discovery-middleware leg in Handler(), and the discovery Stop() call.
• Update affected tests (drop the mockDiscoveryMgr setup, swap routers) and the comments that referenced the removed code (including the e2e circuit-breaker note and the core filterHealthyBackends doc, now the single source of truth for health filtering).

Net: +86 / −2315 across 25 files (1941 lines are whole-file deletions).

Part of #5621

Type of change

• Code cleanup or refactoring

Test plan

• go build ./... — clean
• go vet ./... — clean
• golangci-lint on the changed packages — 0 issues
• go test -race on all affected packages (pkg/vmcp/server, cli, router, core, aggregator, session, test/integration/vmcp) — all pass, including the goleak-guarded suites (no goroutine leaks)

Does this introduce a user-facing change?

No.

Special notes for reviewers

• API break for external embedders: server.New drops its discovery.Manager parameter (7 → 6 args). In-tree callers (cli/serve.go, tests, the integration helper) are updated here; an out-of-tree embedder must drop that argument. The New doc comment documents the change.
• No behavior change on the live path: the discovery middleware's filterHealthyBackends was a copy of the core's own (pkg/vmcp/core/core_vmcp.go); circuit-breaker / unhealthy-backend filtering is unchanged and still covered by TestFilterHealthyBackends.
• Out of scope (pre-existing): the convertAnnotations doc comment in pkg/vmcp/core/admission.go references a server.convertAnnotations copy that was already removed by an earlier stage; left for a focused follow-up. The vestigial Config.AuthzMiddleware field likewise remains for a separate PR.

Large PR Justification

• Removes dead code after vMCP refactor

Generated with Claude Code

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.03%. Comparing base (12fb161) to head (482e34f).

Files with missing lines	Patch %	Lines
pkg/vmcp/cli/serve.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5627      +/-   ##
==========================================
- Coverage   70.04%   70.03%   -0.02%     
==========================================
  Files         652      648       -4     
  Lines       66392    66208     -184     
==========================================
- Hits        46505    46367     -138     
+ Misses      16544    16503      -41     
+ Partials     3343     3338       -5     

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[github-actions]

✅ Large PR justification has been provided. The size review has been dismissed and this PR can now proceed with normal review.