Replace per-client pairing/scope/revocation with a flat model: enrollment establishes the long-term key K once, and the CLI and every MCP agent inherit it over the enrolled channel. clientId becomes an audit-only label.

- remove client store, cached MCP identity, per-client pairing/sync/revoke
- rename `sctl pair` -> `sctl connect`; control /pair-ext -> /enroll
- `sctl mcp` drops the pair subcommand; --name is now an audit label
- mcpserver exposes all tools (no scope filter)
- add an Origin whitelist (chrome-extension:// etc.) as a cheap web pre-filter; the handshake stays the real gate
- source reads are no longer CLI-exempt (source-read policy gates both)
- rewrite protocol.md / threat-model.md / architecture.md / README / AGENTS

…pair/client

Mirror the extension-side protocol change byte-for-byte: envelopeTypes becomes {session, bridge} and the flat-trust-dead pair.*/client.* types are removed. protocol.go's EnvelopeTypes field becomes a nested struct so Load() parses the new shape. protocolVersion unchanged (no wire-format change).