Make agent identities unambiguous and names unique per owner #118

Problem

Agent assignment previously depended on ambiguous names. Duplicate names within one account or common names across owners could not round-trip reliably, and editing a connection label did not rename the underlying identity.

Delivered behavior

• Stable opaque public agent references are independent of names, owners, credentials and connections.
• Active Agent names are unique within one owner after trimming, Unicode Form KC and invariant-case normalization; different owners may use the same name.
• Shared-project roster, assignments and attribution include safe owner context for external Agents without exposing private email addresses.
• Owners can rename the actual Agent identity without changing references, credentials, permissions, assignments or provenance.
• Agent identity names and credential/client labels are distinct in UI and data contracts.
• Fresh OAuth connections create distinct Agents and let the human replace the client-suggested name.
• Deleting an Agent's final credential retires the identity and releases its name while preserving historical task and memory attribution.
• Existing credential-less identities are retired by migration; identities are never silently merged.

Acceptance

• Same-owner duplicates are rejected consistently at creation and rename, including races and OAuth paths.
• The same Agent name under different owners remains valid and unambiguous.
• Public roster and task detail return stable references suitable for subsequent writes.
• Shared-project UI and MCP include safe owner context.
• Rename and migration preserve existing assignments and provenance.

Delivered

Subtasks #120, #121, #123 and #124 are complete. #122 is complete after #191 was canceled as superseded by the owner's final deleted-identity policy.

Evidence: ee81296, e9e0a52, 741c845, af19965; deployed successfully. Full Web test suite: 63 passed, 0 failed. The old ambiguity cases in #105/#115 were rechecked without changing their assignees.