feat(implement): add recipes for scheduler, tenancy, rate-limiting, memory, and provider APIs

[slin1237]

Problem

Five subsystems built since the guide was written (priority scheduler, multi-tenancy, rate-limiting, memory, and the Anthropic/Gemini provider routers) had no step-by-step recipe — they were only listed in the "subsystems without a recipe yet" table in implement/SKILL.md.

Solution

Wrote five new recipes, each by reading the current code for the subsystem and modeling examples on real implementations (every path/type/method/command verified against source), then wired them into the detection table.

Changes

New recipes (feat):

• scheduler-feature — configure the priority scheduler (classes, tenant policies, reservations via PrioritySchedulerYaml) + extend the ClassQueue / TenantPolicyResolver traits
• tenancy-feature — read the resolved TenantKey/RouteRequestMeta to make a feature tenant-aware, and add a new TenantIdentity source
• rate-limit-feature — the single global TokenBucket (concurrency cap + rate limit), how to tune it, and how to add a knob
• memory-feature — concise guide for the MemoryExecutionContext/MemoryPolicyMode parse-decide layer; honestly flagged as in-progress (the execution consumers are still TODO stubs)
• provider-api — add a provider-compatible API surface (RoutingMode variant + RouterTrait impl + factory wiring + route), modeled on AnthropicRouter

implement/SKILL.md: moved these five from the "no recipe yet" table into the detection table (kept Responses/Conversations/Realtime, the skills subsystem, the TUI, and client-gen as still-uncovered).

Metadata (docs): bumped recipe count 15 → 20 in the README recipe table + "Implementation Recipes" line and the "(with 20 subsystem recipes)" string in the Claude plugin, marketplace, and Cursor manifests.

Affected Skills

• smg:map
• smg:contribute
• smg:review-pr
• smg:implement
• Recipe files (5 new: scheduler, tenancy, rate-limit, memory, provider-api)
• Command router (smg.md)
• Plugin config (plugin.json, marketplace.json, cursor)

Checklist

• Follows conventional commit title format
• Commit(s) are signed off (git commit -s)
• Tested with claude plugins add . locally
• Skill triggers correctly on intended prompts
• No broken @file.md references in skills

Verification: load-bearing symbols for all five recipes confirmed present in smg@a18d64f5; all cargo commands use -p smg (the model_gateway package); JSON manifests validated. Doc skeletons weren't cargo-compiled in isolation, so the "tested locally" boxes are left unchecked. The memory recipe is intentionally a "how it works + extension points" guide, since that subsystem is genuinely in-progress in the codebase.