v0.116.0: observability pass — always-on debug ring + leveled sweep + agent tab + self-log pull — MinAgent: 0.81.0

Capture layer: LogBuffer always exists; logger = MultiWriter(LevelFilterWriter
(stdout, logging.level), ring) so DEBUG detail exists remotely without a config
flip while docker logs keep respecting the level. New internal/logx leveled
helpers. Report ACK gains controller_log_requested (additive); next report
ships controller_log_tail (128KB, consume-once, app-tail wire byte-compatible).
Debug page: Vezérlő|Ügynök tabs; agent tab proxies agent /debug/logs with the
pre-0.83 notice on typed 404. Sweep: netstorage_job phases, netprobe, handler
validation refusals + orphan WARN, SupportsWithSource gate line, agentapi
per-call DEBUG, migrate phase lines, tier2/offbox unswallowed persists.
Red-proofs: filter-disabled, drain-removed, dropped-phase-line all FAIL.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

controller/internal/agentapi/features.go

@@ -7,6 +7,7 @@ import (
 	"sync"
 	"time"
 
+	"gitea.dooplex.hu/admin/felhom-controller/internal/logx"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/util"
 )
 
@@ -109,13 +110,21 @@ type SupportCache struct {
 // window, probe on miss). The probe runs OUTSIDE the lock — concurrent misses may double-probe
 // (harmless: the probe is one cheap GET).
 func (sc *SupportCache) Supports(ctx context.Context, p SupportProber, f Feature) SupportState {
+	state, _ := sc.SupportsWithSource(ctx, p, f)
+	return state
+}
+
+// SupportsWithSource is Supports plus the DECISION SOURCE ("version" | "probe-cache" |
+// "probe" | "unregistered") — the v0.116.0 observability extension so the gate line can
+// say HOW the verdict was reached. Behavior is byte-identical to v0.115.0 Supports.
+func (sc *SupportCache) SupportsWithSource(ctx context.Context, p SupportProber, f Feature) (SupportState, string) {
 	probe, ok := featureProbes[f]
 	if !ok {
-		return SupportUnknown // unregistered feature — never refuse on a table gap
+		return SupportUnknown, "unregistered" // unregistered feature — never refuse on a table gap
 	}
 	if vr, hasVer := p.(AgentVersionReporter); hasVer {
 		if state, decided := supportsByVersion(vr.AgentVersion(), f); decided {
-			return state
+			return state, "version"
 		}
 	}
 	sc.mu.Lock()
@@ -125,7 +134,7 @@ func (sc *SupportCache) Supports(ctx context.Context, p SupportProber, f Feature
 	}
 	if e, hit := sc.entries[f]; hit && nowFn().Sub(e.at) < supportTTL {
 		sc.mu.Unlock()
-		return e.state
+		return e.state, "probe-cache"
 	}
 	sc.mu.Unlock()
 
@@ -138,14 +147,17 @@ func (sc *SupportCache) Supports(ctx context.Context, p SupportProber, f Feature
 		sc.entries[f] = supportEntry{state: state, at: nowFn()}
 		sc.mu.Unlock()
 	}
-	return state
+	return state, "probe"
 }
 
 // Supports probes (cached, TTL 5m, both polarities) whether the connected agent provides the
 // feature. 2xx ⇒ Yes. 404 ⇒ No. Anything else ⇒ Unknown (never "too old"). The web layer drives
 // the same machinery through its netAgent seam (Server.netFeatures) so tests can fake the probe.
 func (c *Client) Supports(ctx context.Context, f Feature) SupportState {
-	return c.features.Supports(ctx, c, f)
+	state, source := c.features.SupportsWithSource(ctx, c, f)
+	logx.Debugf(c.logger, "[agentapi] Supports(%s) = %s (source=%s agent_version=%q)",
+		f, state, source, c.AgentVersion())
+	return state
 }
 
 // supportsByVersion decides a feature by version comparison alone. decided=false (unknown/garbage