feat(memory): retrieve-inject long-term memory at turn start (closes #154)

[dahai80]

Closes #154. Completes the retrieve half deferred by #151 (which landed turn-end commit only). fusion-cowork #68 and fusion-agent-studio #247 already have both commit+retrieve; this brings fusion-code to parity.

What

At main-agent turn start (processUserInput), recall relevant cross-session memory from fusion-memory (fm-server) and inject it as an attachment message on the same path as UserPromptSubmit hook additionalContext. The agent starts each turn with relevant prior context, not stateless.

Why now

#151 deliberately wired only the commit half — its body noted the subagent spawn path (AgentTool.tsx) was the wrong injection seam for retrieve and deferred the correct main-agent seam to a follow-up issue (#154). The correct seam is the user-input processing path (processUserInput), which runs per-turn for the main conversational agent and already collects additionalContext from UserPromptSubmit hooks. This PR injects memory there.

How

• src/services/memory/retrieveContext.ts (new) — retrieveMemorySection(): calls the existing retrieveContext RPC + formatContextToPrompt (both added by feat(memory): integrate fusion-memory long-term memory commit #151 but unwired), returns a <fusion_memory_context> section string or "". Main-agent guard (!agentId), fail-empty (logForDebugging, never throws). Env override: FUSION_MEMORY_RETRIEVE_TOP_K (default 10) / FUSION_MEMORY_RETRIEVE_BUDGET (default 4096).
• src/utils/processUserInput/processUserInput.ts (edit) — after UserPromptSubmit hooks, on the happy path (result.shouldQuery) with real input and !context.agentId, fire-and-forget retrieveMemorySection and push the section as a hook_additional_context attachment message. Mirrors the existing hook additionalContext push. Lazy-require the module (same pattern as feat(memory): integrate fusion-memory long-term memory commit #151's stopHooks) to avoid circular deps.

Wire contract (already implemented in fm-server)

JSON-RPC 2.0: retrieve -> {blocks: ContextBlock[], total_tokens: number}. HTTP POST /v1/memory/retrieve (Bearer auth). Full contract: fusion-memory/clients/README.md.

Degradation

fm-server down / FUSION_MEMORY_API_KEY unset / no memory matched -> section is empty, nothing injected, debug log only. Never blocks the main loop. 100% offline — HTTP only to 127.0.0.1.

Config (operator env, same as #151)

Env	Default	Purpose
FUSION_MEMORY_BASE_URL	http://127.0.0.1:11440	fm-server base URL
FUSION_MEMORY_API_KEY	(none)	Bearer token; if unset, retrieve is skipped
FUSION_MEMORY_RETRIEVE_TOP_K	10	top-k memories to recall
FUSION_MEMORY_RETRIEVE_BUDGET	4096	token budget for retrieved context

Tests

10 tests in retrieveContext.test.ts (subagent skip, empty input, success format, RPC error, HTTP non-2xx, fetch throw, missing API key, no-match, top_k env override, invalid env fallback). Full memory suite 25 pass, bun run build green (verified against current main incl. #155).

Scope

• New: src/services/memory/{retrieveContext,retrieveContext.test}.ts
• Edit: src/utils/processUserInput/processUserInput.ts (import + inject block, lazy-require module)
• No changes to existing session memory, extract-memories, KB, auto-dream, or the commit path (feat(memory): integrate fusion-memory long-term memory commit #151).