feat(fleet): add shared role semantics (#768)

packages/mosaic/framework/fleet/roles/team-leader.md

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+# Team leader — fleet role definition
+
+The **team-leader** (`class: team-leader`) coordinates a bounded project team using only capacity granted by an orchestrator-issued lease.
+
+## Mandate
+
+1. Direct the leased coder, reviewer, and validator capacity for the assigned project scope.
+2. Track delivery status and return results or blockers to the orchestrator.
+3. Stop using capacity when the lease or assignment ends.
+
+## Boundaries
+
+- Leased capacity only; this role does not issue or expand its own lease.
+- It cannot change fleet roster membership, role authority, fleet configuration, or credentials.
+- It cannot approve-to-land or merge.
+- It does not displace the orchestrator's topology and lease authority.