feat(backend): implement real HTTP dispatch for agent endpoints

[samsolo247]

Summary

Closes #169

Replaces the broken defaultDispatch stub in app.ts — which always threw, causing every submitted task to immediately fail — with a real HTTP dispatch implementation wired through the agent registry.

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Changes

backend/src/coordinator/dispatch.ts (new)

• httpDispatch() sends a DAG node to an agent via POST <endpoint>/execute
• Configurable timeout via AbortController (default 60s)
• Exponential backoff retry (default 3 retries, 1s base delay)
• Retryable on 5xx and 429; immediate failure on non-retryable 4xx
• Injectable fetch implementation for full testability

backend/src/api/app.ts (modified)

• makeHttpDispatch(registry) replaces the throwing stub
• Resolves the cheapest available agent for each node type via the registry
• Falls through to a clear misconfiguration error when no registry is provided

backend/src/coordinator/dispatch.test.ts (new)

• Successful dispatch returns parsed JSON
• POSTs to <endpoint>/execute with correct headers and body
• Empty body returns {}
• Trailing slash stripped before appending /execute
• Retries on 503, 429, and ECONNREFUSED
• Does not retry on 4xx (400, 404)
• Throws after all retries exhausted
• Timeout wrapped as descriptive error; counts toward maxRetries

backend/tests/e2e/pipeline.test.ts (modified)

• Added HTTP dispatch integration (mock agent server) suite
• Real in-process HTTP server acts as a mock agent
• Wires into createApp via agentRegistry only (no custom dispatch)
• Asserts all 5 DAG nodes complete via real HTTP calls
• Asserts task fails correctly with no registry and no dispatch

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Test results

PASS src/coordinator/dispatch.test.ts
PASS tests/e2e/pipeline.test.ts

All acceptance criteria from #169 satisfied.

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