Feat: Human-in-the-loop effect for APL and elicitation plugin

[terylt]

Summary

Adds human-in-the-loop (HIL) elicitation to APL: a policy can pause an operation to ask a human — manager approval, a confirm, a step-up re-auth, an attestation — and resume once the human responds, without blocking the request path. The human's decision is bound to the actual request args and recorded from CPEX-owned state, so it is the source of truth for "what was approved," never an LLM summary. Includes a working Keycloak CIBA channel plugin.

A policy expresses it with a sugar verb that parses to a single Step::Elicit:

plugins:
  - name: manager-approver
    kind: elicitation/ciba
    hooks: [elicit]

routes:
  - tool: approve_raise
    pre_invocation:
      - "require_approval(manager-approver, from: claim.manager, channel: \"ciba\", scope: \"args.amount <= 25000\", purpose: \"Approve raise\")"

from is who to ask (approver, resolved from the bag — may differ from the subject); scope is the args-binding the runtime checks against the live request; purpose is the audited, human-readable description.

The model: async, retry-based resume

An elicitation has three short, synchronous touch-points — the hours-long human gap lives in the channel (Keycloak CIBA), never in a blocking call:

1. Dispatch — first arrival: register the intent, open the channel backchannel, return a correlation id.
2. Check — on each agent retry: read status (Pending / Resolved{approved|denied} / Expired) without blocking.
3. Validate — once resolved: verify the response is genuine (signed token, intent binding, responder identity). The runtime then layers the scope-over-args sufficiency check before honoring the approval.

While pending, the phase suspends rather than denies. Decision stays binary — a suspended phase reports Allow + Some(PendingElicitation), and the host maps that to JSON-RPC -32120 ("not complete, retry echoing this id"). The forwarding rule stays one clause: forward iff Allow AND pending.is_none(). Expiry / channel error fails closed (on_error: deny default).

What's included

Area	Change
DSL verbs (apl-core)	require_approval / confirm / step_up / attestation / info / review — sugar over one Step::Elicit(ElicitStep) with an ElicitKind. Fields: from, purpose, scope, timeout, channel, config_override, on_error. Parser + evaluator (dispatch/check/validate flow, pending short-circuit, scope-over-args) + route.rs resume wiring.
Elicitation hook family (cpex-core)	New ElicitationHook (single hook name "elicit") + ElicitationPayload with a Dispatch/Check/Validate operation discriminator — mirrors the delegation hook layout; dispatch is free via invoke_entries::<ElicitationHook>. cpex-core keeps its own ElicitationOp/ElicitationStatusKind/ElicitationOutcomeKind (no apl-core dep); the bridge maps.
Bridge / invoker (apl-cpex)	ElicitationInvoker (dispatch/check/validate) + elicitation_invoker.rs; dispatch-plan resolves an elicit entry by name; route_handler drives the pending/resume flow and surfaces elicitation.* bag attrs (id, approver, intent_id, channel) for audit.
CIBA channel plugin (builtins/plugins/elicitation-ciba, new crate)	kind: elicitation/ciba — a HookHandler<ElicitationHook> implementing the three operations against Keycloak CIBA: from → login_hint, purpose → binding_message, timeout → requested_expiry. Intent store, config, factory. Registered via cpex-builtins.
Args binding via scope	The approval is bound to the request with an APL boolean expression (args.amount <= 25000) the runtime evaluates at validate — kept in APL because Keycloak has no RFC 9396 RAR.

Notable design decisions

• Approver ≠ subject. from resolves the party to ask from the request bag (e.g. claim.manager); the resolved identity is cross-checked against the actual responder at validate.
• Sufficiency vs genuineness split. The channel plugin proves the response is genuine (signature / intent binding / responder); the sufficiency check (scope over live args) stays in the runtime because it is an APL expression the plugin can't evaluate.
• One hook, three operations. A single elicit hook + payload discriminator (not three hook names) keeps registration and the dispatch plan trivial — one plugin, one entry, resolved by name like token.delegate.
• Tri-state without widening Decision. PendingElicitation rides alongside an Allow so the deny path stays untouched; the host's one-clause forward rule and -32120 retry contract carry the suspend/resume.
• Fail-closed. Expiry, channel error, or a failed validation deny (subject to on_error).
• cpex-core decoupled from apl-core. The hook payload defines its own op/status/outcome enums; the apl-cpex bridge translates to/from apl-core's ElicitKind / ElicitationStatus / ElicitationOutcome.

Public API surface

• cpex-core::elicitation: ElicitationHook, HOOK_ELICIT ("elicit"), ElicitationPayload, ElicitationOp, ElicitationStatusKind, ElicitationOutcomeKind.
• apl-core::step: Step::Elicit, ElicitStep, ElicitKind, ElicitationInvoker (dispatch/check/validate), ElicitationDispatch, ElicitationStatus, ElicitationOutcome, ElicitationValidation, PendingElicitation, ElicitationError.
• apl-cpex: ElicitationInvoker bridge, dispatch-plan elicitation_entries, route-handler pending/resume wiring.
• builtins/plugins/elicitation-ciba: kind = "elicitation/ciba" (CibaApproverFactory), registered through cpex-builtins.

Testing

• CIBA plugin — builtins/plugins/elicitation-ciba/tests/ciba_e2e.rs (7 tests) + live_keycloak.rs (1 live integration test, ignored by default).
• apl-cpex — tests/elicit_step_e2e.rs: end-to-end through a real PluginManager (dispatch → pending → resolve → validate; approve / deny / expire; args-binding). Existing suites (delegate_step_e2e, end_to_end_route, cmf_invoker_dispatch) updated for the new step/plan shape.
• apl-core — parser / evaluator / step.rs / route.rs unit tests; tests/yaml_end_to_end.rs updated for the elicit verbs; apl-cmf/tests/end_to_end.rs touched.

Note: I have not re-run the full workspace suite on this branch — worth a cargo test --workspace / make test befor

Follow-ups

• Site docs page for elicitation under docs/content/docs/apl/ (none in this PR).
• Additional channel plugins beyond CIBA (Slack, in-band); capability-gating for the elicit hook; richer per-kind validation contracts.