feat(go): full endpoint editability via MCP, with validation

[rado0x54]

What

Makes shellwatch_manage_endpoints create/update accept every REST-editable field — userVerification, agentForward, label, host, port, username, description — with proper validation, in both backends. Full endpoint editability via MCP is the intended contract; the tool schema silently stripping userVerification/agentForward was a bug, not the spec.

Go changes

• internal/agent/session.go — UpdateEndpoint now takes a typed EndpointPatch (nil = keep, DescriptionSet tri-state for null-clears) instead of map[string]any, keeping MCP wire shape out of the session-isolation type.
• internal/mcp/tools_endpoints.go — new endpointPatchFromWire decode/validate layer (the zod equivalent): userVerification enum, agentForward boolean, description ≤1000 chars or null, field type checks. Tool data input schema now declares the properties (incl. the enum) so agents can discover them.
• Create parity fix — MCP create previously inserted agentForward=false; it now defaults true like the Node repo and REST. Explicit userVerification/agentForward on create are honored (validated), defaults unchanged (required/true/port 22).

Node changes

• src/mcp/tools/endpoints.ts — the zod data schema gains userVerification (enum) and agentForward (boolean); create passes them through (repo defaults required/true still apply when absent). Update needed no repo changes — the repository already accepted the fields; only the tool schema stripped them.

Docs

• docs/api/mcp-tools.md documents the full data shape, create defaults, and validation-rejects behavior.
• docs/api/README.md item C notes the former stripping as a resolved bug in both backends.

Tests

• internal/mcp/tools_endpoints_test.go — decode/validation unit tests (valid full patch, null-clear, enum/type/length rejections).
• internal/agent/session_endpoint_test.go — typed patch merge incl. userVerification + description set/clear.
• internal/httpserver/mcp_test.go::TestMCPEndpointFullEdit — Go end-to-end over the real streamable-HTTP transport: create with non-default fields, update them, invalid enum rejected without mutating the row, create defaults verified.
• src/mcp/server.test.ts — Node end-to-end: create/update with the new fields, read-back, invalid enum rejected without writing.

go vet ./..., go test ./..., pnpm typecheck, pnpm lint (changed files), pnpm vitest run src/mcp/server.test.ts, pnpm spdx:check all pass; mcp-* goldens untouched (create/update were never golden-pinned).