🤖 perf: reuse agent definitions within stream requests - #3954

Summary

Reuses parsed agent-definition packages within a single stream request so agent resolution, inherited body/frontmatter construction, and available-subagent discovery do not repeatedly read the same winning definition file.

Implementation

• Adds a request-owned AgentDefinitionRequestCache keyed by the concrete runtime and normalized source path, with winning lookups recorded only after successful parsing.
• Preserves same-scope root precedence and invalid-candidate fallback across .xum, .mux, host-global, and plugin sources.
• Carries plugin attribution through cached descriptor construction.
• Wires reuse through agent resolution, inheritance, stream-context body/frontmatter resolution, and available-subagent discovery.
• Adds end-to-end regression coverage for project → global → built-in inheritance, shadowing, invalid same-scope candidates, disabled filtering, desktop gating, inherited runnable state, and tool policy. The selected definition is read four times without the seam and once with request-scoped reuse.

Validation

• bun test src/node/services/agentResolution.test.ts src/node/services/agentDefinitions/agentDefinitionsService.test.ts src/node/services/streamContextBuilder.test.ts src/node/services/aiService.test.ts — 144 passed
• Prettier write/check for all touched files
• git diff --check
• Fresh read-only review: no Critical or Important findings

make build-main, targeted ESLint, and bun run tsgo --noEmit -p tsconfig.main.json were attempted repeatedly but the local processes were killed with exit 137/SIGKILL in the constrained workspace.

Risks

This touches agent precedence and inheritance plumbing. The cache is intentionally request-scoped and only stores successfully parsed winners by concrete source identity; existing fallback, disablement, desktop capability, and policy behavior remains covered by the integration test and existing service suites.

Generated with xum • Model: openai:gpt-5.6-sol • Thinking: medium • Cost: $unknown