Align contacts with organization-wide access

[alex-w-99]

Decisions

• Plugin surface: Kept the existing contact CRUD tools because they remain compatible, while replacing obsolete per-identity visibility guidance with organization-wide behavior.
• SDK compatibility: Raised the SDK floor to 0.5.1 and pinned contract CI to the SDK 0.5.1 source revision.

Changes

• Update prompts, tool comments, setup requirements, diagnostics, and documentation.
• Explain that all identities can read contacts and mutations affect the shared address book.

Verification

• pytest -q against SDK 0.5.1 source (282 passed, 22 skipped)

Related Changes

• SDK: SDK 0.5.1: add contact memory inkbox#119
• OpenCode: Align contacts with organization-wide access opencode-plugin#8
• OpenClaw: Align contacts with organization-wide access openclaw-plugin#42
• Hermes: Align contacts with organization-wide access hermes-agent-plugin#70
• Codex: Align contacts with organization-wide access codex-plugin#25