Remove dead session-factory aggregation and composite mirrors

[tgrunnagle]

Summary

With the core established as the single source of capability aggregation, composite-tool
execution, and workflow telemetry on the Serve path (#5445), the session factory's parallel
aggregation/composite/telemetry machinery is unreachable dead code. This is Stage 2 of the
#5621 dead-code removal.

• Aggregation mirror (ce4dce6a): remove session.WithAggregator, the factory aggregator
  field + buildRoutingTableWithAggregator, and Aggregator.ProcessPreQueriedCapabilities
  (interface + impl + regenerated mock). makeBaseSession always builds the routing table from
  raw backend capabilities (the core owns overrides/conflict-resolution/filtering); its
  now-always-nil error return is dropped. cli/serve.go drops the WithAggregator(agg) wiring
  (agg still feeds the core via Config.Aggregator).
• Composite/telemetry mirror (549caf74): remove compositeToolsDecorator and the
  workflowExecutorInstruments family from the session manager, plus the dead
  FactoryConfig.WorkflowDefs/ComposerFactory fields. The core owns composites
  (executeComposite) and workflow telemetry (telemetryComposer, Forward passthrough headers on the Serve path #5561). The optimizer
  decorator and its telemetry are untouched.

Verified dead: nothing sets FactoryConfig.WorkflowDefs/ComposerFactory (production and the
integration harness leave them unset), and ProcessPreQueriedCapabilities's only caller was the
removed factory aggregation.

Part of #5621.

Type of change

• Refactoring (no behavior change)

Test plan

• Unit tests (task test)
• Linting (task lint)
• Integration tests (go test ./test/integration/vmcp/...)

Build clean; pkg/vmcp/... + test/integration/vmcp/... pass; lint clean except the
pre-existing, unrelated G115. Pure deletion of unreachable code (10 files, +14/−809).

API Compatibility

session.WithAggregator and Aggregator.ProcessPreQueriedCapabilities are removed from the
exported surface; their only in-tree users were the factory aggregation path (removed here) and
cli/serve.go. server.New's signature is unchanged — the discoveryMgr-param change is
Stage 3 (#5621).

Does this introduce a user-facing change?

No.

Special notes for reviewers

Stage 2 of 3 in the #5621 dead-code removal (follows the merged Stage 1, #5622). The
optimizer decorator and its telemetry (monitorOptimizer/telemetryOptimizer) are intentionally
retained — out of scope here. Stage 3 removes the discovery middleware + context seam, untangles
default_router, and changes the server.New signature (7→6, dropping discoveryMgr).

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.13%. Comparing base (799b422) to head (549caf7).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5625      +/-   ##
==========================================
+ Coverage   69.93%   70.13%   +0.20%     
==========================================
  Files         650      652       +2     
  Lines       66289    66277      -12     
==========================================
+ Hits        46358    46484     +126     
+ Misses      16588    16434     -154     
- Partials     3343     3359      +16     

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