docs: prepare mcp source discovery loop

docs/source-discovery/README.md

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+# Source Discovery Ledger
+
+Date: 2026-07-09
+Current loop: Stage B1 - N12-pre zyl evidence source discovery
+
+## Objective
+
+Find the best real capability source for the MCP core read tools before implementing Stage C.
+
+## Inputs
+
+- `runs/offline-evidence-intake/zyl-qqfile-20260709/archives/zyl.rar`
+- `runs/offline-evidence-intake/zyl-qqfile-20260709/metadata/archive-list.txt`
+- `runs/offline-evidence-intake/zyl-qqfile-20260709/metadata/sha256.txt`
+- `docs/mcp-core-tools-contract.md`
+- `docs/mcp-core-business-plan.md`
+
+## Loop cadence
+
+1. Inventory evidence.
+2. Identify source candidates.
+3. Map candidates to MCP tools.
+4. Select primary/fallback source per tool.
+5. Open one Stage C implementation slice.
+6. Verify with `go test ./...`, `go build ./cmd/isphere-mcp`, and MCP verification script.
+7. Commit exact paths.
+
+## Current target order
+
+1. `isphere_receive_messages`
+2. `isphere_search_contacts`
+3. `isphere_search_groups`
+4. `isphere_receive_files`
+
+Reason: message source discovery is the current blocker; contacts/groups can be derived or selected after the message source shape is known.
+
+## Current plan
+
+See `docs/superpowers/plans/2026-07-09-mcp-core-source-discovery-loop.md`.