fix: drop removed smg subsystems (Skills/memory/blob_storage), add least_load policy

[slin1237]

Problem

21 commits landed in smg since the dev-guide's last verification baseline (a18d64f5 → 27b528f3). Several removed subsystems the guide documents, and one added a routing policy:

• 🗑️ Skills API — crates/skills, crates/protocols/skills.rs, routers/skills (smg#1612)
• 🗑️ blob_storage crate (backed Skills)
• 🗑️ conversation memory — model_gateway/src/memory/ + the data_connector conversation-memory writer/background (smg#1613, #1631)
• ➕ least_load load-balancing policy → 9 policies now (smg#1629; weight param renamed kv_pressure_weight in #1632)
• ➕ MeshAdapters gateway composition root (mesh/wiring.rs, smg#1626)

Solution

Sync the guide to the current code: delete the stale recipe, drop the removed subsystems from map/implement/README/manifests, bump the policy count, trim the now-3-field StorageBundle, and de-collide the routing-policy example.

Changes

• delete skills/implement/memory-feature.md (subsystem removed)
• map: remove skills + blob_storage crate rows, the Memory subsystem row, and "Skills" from the provider-routers list; 8 → 9 routing policies (overview + subsystems table)
• implement/SKILL.md: drop the memory detection-table row and the Skills "no recipe yet" row
• storage-backend.md: StorageBundle is now three handles — removed conversation_memory_writer + background_repository and all ConversationMemoryWriter references
• routing-policy.md: 8 → 9 policies (add least_load); rename the hypothetical example least_loaded → my_policy so it no longer collides with the new real least_load policy
• mesh-feature.md: note the MeshAdapters gateway composition root (model_gateway/src/mesh/wiring.rs) — engines registered before gossip
• README + 3 plugin manifests: 20 → 19 recipes

Affected Skills

• smg:map
• smg:contribute
• smg:review-pr
• smg:implement
• Recipe files (deleted memory-feature; updated storage-backend, routing-policy, mesh-feature)
• 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

Verified against smg@27b528f3: removed-subsystem symbols confirmed absent, the in-memory storage backend (HistoryBackend::Memory) deliberately kept, mesh-feature's "keyed by key not by replica" claim re-checked against #1592 (still accurate), JSON manifests validated.