refactor(prompt): extract resolvePart from createUserMessage as sibling Effect.fn

[ozgurulukir]

Summary

• Move the 265-line nested resolvePart closure out of createUserMessage to a sibling Effect.fn (resolveUserPart) inside the SessionPrompt layer
• The function takes a ResolveUserPartCtx parameter carrying the per-message context (input, info, agentPermission), keeping closure access to shared services (mcp, fsys, registry, lsp, provider, bus)
• Hoist DraftPart<T> and ResolveUserPartCtx types to module scope so both createUserMessage (via assign) and resolveUserPart can reference them

Why

createUserMessage was a 367-line function with a 265-line nested Effect.fn (resolvePart) that handled file/agent/text part resolution. The nesting prevented testing in isolation and made the closure dependencies implicit. Extracting to a sibling keeps the same closure access to layer-scoped services while making the per-message context explicit.

Verification

• bun typecheck (tsgo --noEmit) passes
• bun test test/session/prompt.test.ts: 10/10 pass
• Full session suite (577 tests): no new failures vs baseline (19 pre-existing failures identical to baseline, modulo timing)
• Behavior is unchanged — body is byte-identical except for Draft< → DraftPart< rename and ag.permission → ctx.agentPermission

Test plan

• bun typecheck
• bun test test/session/prompt.test.ts
• bun test test/session/ (diff against baseline)