feat: read agent_id from ctx (SDK now plumbs it)

Plexum-sdk-go now unpacks `_meta.agent_id` from host's MCP tools/call
frames into the ctx. Plugin reads it via plugin.AgentIDFromContext
and uses it for backend api-key resolution (agentKeys map lookup +
bearer token) and HF on_call pre-check.

config.defaultAgentID demoted from "v1 stop-gap for the missing SDK
plumbing" to "fallback for operator-driven plugin-call". Normal turn
loops carry the real caller id through ctx now; defaultAgentID only
fires on host-driven dispatch paths.

Also: log every CallTool dispatch with {tool, agent_id} at info level
so operators can see which agent is hitting which tool without
debug-level chatter.

E2E in sim: PLEXUM_ECHO_FORCE_TOOL_USE drives echo provider →
dialectic_list_topics dispatched via host → plugin sees
agent_id=test-agent in ctx + logs it correctly.

README.md

@@ -59,18 +59,20 @@ Restart the host afterwards: `systemctl --user restart plexum`.
 | `agentKeys` | `{}` | Per-agent bearer token overrides. |
 | `defaultAgentID` | — | Agent id reported to the backend when host hasn't surfaced one via tool ctx. |
 
-## How agent identity is resolved (v1 limitation)
+## How agent identity is resolved
 
 `Dialectic.OpenclawPlugin` got the calling agent's id via the OpenClaw
-framework's `ctx.agentId`. The Plexum SDK doesn't yet surface this on
-tool dispatch — same constraint `HarborForge.PlexumPlugin` hits. v1
-falls back to `config.defaultAgentID`. Multi-agent claws can configure
-`agentKeys` but the bearer token used per call is selected against the
-config default (not the true caller), which is fine for a homogeneous-
-role claw but won't sort signed verdicts apart by agent.
+framework's `ctx.agentId`. The Plexum SDK now propagates the same
+information: the host attaches `_meta.agent_id` to every MCP
+`tools/call` frame it dispatches on an agent's behalf, and SDK
+serve.go unpacks it into the ctx. Plugins read it via
+`plugin.AgentIDFromContext(ctx)`.
 
-The fix (deferred): plumb `AgentContext` through `ToolPlugin.CallTool`
-in `Plexum-sdk-go`. Once landed, swap `AgentIDFromCtx` to read it.
+`config.defaultAgentID` remains as a fallback used only on host paths
+that genuinely carry no agent context — operator-driven
+`plexum plugin-call` invocations for debugging, etc. Leave it empty
+in a normal deployment; tool calls coming from real turn loops will
+always carry the caller agent id.
 
 ## HF on_call coverage pre-check
 
@@ -86,7 +88,6 @@ to make the skip explicit (matches the OpenClaw plugin escape hatch).
 
 ## Deferred items
 
-- **Per-call agent id** — see "How agent identity is resolved" above.
 - **HF window-coverage check** — needs a backend-side endpoint or a
   Plexum cross-plugin contract for `harbor-forge` to surface
   `HasOnCallCovering(agentID, from, to)`.