Add generic host-verified /webhook/tools/{tool} ingress (#757)

* Add generic host-verified webhook ingress for tools

* Stabilize trace E2E test rig and approval behavior

* Fix webhook security issues from review feedback

- Reject tools without webhook_capability() (was unauthenticated RCE)
- Remove secret-in-query-string fallback (leak via logs/referrers)
- Require approval for event_emit tool (escalation via routine triggers)
- Simplify header_value() (HeaderMap already case-insensitive)
- Redact internal errors from webhook HTTP responses
- Remove unused hmac_timestamp_tolerance_secs field
- Add regression test for tool without webhook capability

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* Harden webhook ingress: require auth mechanism, body limit layer, health check

- Reject webhook capabilities that declare no auth mechanism (empty
  WebhookCapability would previously allow unauthenticated access)
- Add DefaultBodyLimit layer to reject oversized payloads before buffering
- Health check (GET) now verifies tool has webhook_capability(), not just
  existence
- Add regression tests for all three fixes

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* Fix auto_approve_tools inconsistency between dispatcher and thread_ops

dispatcher.rs skips all approval checks (including Always) when
auto_approve_tools is true, but thread_ops.rs still required approval
for Always tools. This caused deferred tool calls to unexpectedly halt
in test rigs and auto-approve configurations.

Match dispatcher behavior: short-circuit all approval when
auto_approve_tools is enabled.

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/agent/thread_ops.rs

@@ -925,14 +925,20 @@ impl Agent {
 
             for (idx, tc) in deferred_tool_calls.iter().enumerate() {
                 if let Some(tool) = self.tools().get(&tc.name).await {
-                    use crate::tools::ApprovalRequirement;
-                    let needs_approval = match tool.requires_approval(&tc.arguments) {
-                        ApprovalRequirement::Never => false,
-                        ApprovalRequirement::UnlessAutoApproved => {
-                            let sess = session.lock().await;
-                            !sess.is_tool_auto_approved(&tc.name)
+                    // Match dispatcher.rs: when auto_approve_tools is true, skip
+                    // all approval checks (including ApprovalRequirement::Always).
+                    let needs_approval = if self.config.auto_approve_tools {
+                        false
+                    } else {
+                        use crate::tools::ApprovalRequirement;
+                        match tool.requires_approval(&tc.arguments) {
+                            ApprovalRequirement::Never => false,
+                            ApprovalRequirement::UnlessAutoApproved => {
+                                let sess = session.lock().await;
+                                !sess.is_tool_auto_approved(&tc.name)
+                            }
+                            ApprovalRequirement::Always => true,
                         }
-                        ApprovalRequirement::Always => true,
                     };
 
                     if needs_approval {