feat(router): milestone 2 slice 1 session-aware policy (#15)

* feat(router): add session-aware policy inputs and continuity switching

* feat(router): propagate policy metadata and add milestone 2 checkpoint

* fix(router): align continuity metadata and refusal explanations

* docs/router: clarify emitted fields and add fixture hard-constraint case

* fix(router): clarify current-target blocker semantics

---------

Co-authored-by: Hanna Rosengren <4538260+hannasoderstromdev@users.noreply.github.com>

Author: hannasdev

docs/contracts/router-contracts.md

@@ -154,11 +154,32 @@ Notes:
   "selectedTargetId": "string|null",
   "shouldSwitch": "boolean|null",
   "continuityCost": "low|medium|high|null",
+  "continuityDecision": "stay_on_current_target|select_target_without_current_context|avoid_switch_due_to_continuity_cost|switch_target|no_selected_target|null",
+  "continuityReason": "string|null",
   "routingOverride": {
     "requested": "auto|stronger|cheaper|stay",
     "applied": "boolean",
     "reason": "string|null"
   },
+  "modeResolution": {
+    "previousMode": "plan|implement|debug|review|summarize|agent_workflow|out_of_domain|null",
+    "proposedMode": "plan|implement|debug|review|summarize|agent_workflow|out_of_domain",
+    "resolvedMode": "plan|implement|debug|review|summarize|agent_workflow|out_of_domain",
+    "transitionReason": "string"
+  }|null,
+  "policyInputs": {
+    "hardConstraints": {
+      "privacy": "enforced|advisory|off",
+      "availability": "enforced|advisory|off",
+      "clientCompatibility": "enforced|advisory|off",
+      "requiredPrivacyTier": "external|local|standard|restricted|unknown|null",
+      "clientSurface": "string|null"
+    },
+    "softConstraints": {
+      "userPreference": "auto|stronger|cheaper|stay",
+      "projectOverride": "string|object|null"
+    }
+  }|null,
   "explanation": "string",
   "refusalReason": "string|null"
 }
@@ -170,6 +191,7 @@ Notes:
 * `shouldSwitch` and `continuityCost` may be `null` when status is refused.
 * `softConstraintInputs` records preference inputs for explainability even when policy depth is staged.
 * `routingOverride` matches current implementation output naming.
+* `continuityDecision`, `continuityReason`, `modeResolution`, and `policyInputs` are currently emitted by the implementation and treated as additive experimental fields in `0.1.0-experimental`.
 
 ### 5) ContextPackage
 

docs/decision-log.md

@@ -123,3 +123,43 @@ Consequences:
 Follow-up:
 - Next review milestone: Milestone 2 plan-to-implementation checkpoint.
 - Linked artifacts (logs, fixtures, docs, PRs): docs/ROUTER-PHASE-PLAN.md, docs/contracts/router-contracts.md
+
+## Milestone 2 Checkpoint (Slice 1)
+
+Decision ID: DEC-2026-05-10-milestone-2-slice-1-session-policy
+Related deferred item: Milestone 2 staged policy depth
+Status: committed
+Date: 2026-05-10
+Owners: team
+
+Context:
+- Milestone 2 requires session-aware routing with explicit policy inputs, while keeping current behavior stable and deferring high-risk commitments until verification gates are met.
+
+Options considered:
+- Option A: implement full policy depth and enforcement in one large change.
+- Option B: ship a scoped first slice with session-mode resolution, named hard/soft inputs, and continuity-aware switching plus focused tests.
+
+Tradeoffs:
+- Option A: faster feature completion on paper, higher regression and review risk.
+- Option B: lower risk and clearer validation trail, requires additional follow-up slices.
+
+Verification signal:
+- Expected signal from phase plan: policy inputs represented explicitly and decision outputs explainable with session and continuity context.
+- Evidence observed:
+  - Router now exposes `modeResolution`, `policyInputs`, `taskType`, and continuity fields in route decisions.
+  - Switchboard evidence summaries now carry these fields through workflow logs.
+  - Tests cover staged hard constraints (availability, privacy, client compatibility) and continuity decisions.
+
+Decision:
+- Chosen option: Option B.
+- Scope of commitment: complete Milestone 2 slice 1 with deterministic session-policy behavior and evidence propagation.
+- What remains intentionally deferred: numeric continuity scoring, final taxonomy freeze, strict universal enforcement defaults, and cross-surface validation beyond current scope.
+
+Consequences:
+- Near-term implementation impact: improves explainability and policy shape with minimal API disruption.
+- Test and replay impact: adds coverage for staged hard constraints and continuity cost behavior.
+- Migration impact: low; fields are additive and aligned with experimental contracts.
+
+Follow-up:
+- Next review milestone: Milestone 2 slice 2 planning.
+- Linked artifacts (logs, fixtures, docs, PRs): src/router/router.js, src/switchboard/workflow.js, test/router.test.js, test/switchboard-workflow.test.js

src/router/data/fixtures.json

@@ -122,6 +122,26 @@
       "classificationReason": "implementation_signal"
     }
   },
+  {
+    "name": "availability_constraint_refusal",
+    "input": "Implement the plan.",
+    "session": {
+      "mode": "plan",
+      "policyInputs": {
+        "hardConstraints": {
+          "availability": "enforced"
+        }
+      }
+    },
+    "targetOverride": "coder_unavailable",
+    "expected": {
+      "status": "refused",
+      "mode": "implement",
+      "requiredCapabilities": ["repo_context", "file_read", "file_edit"],
+      "explanationIncludes": ["hard constraints"],
+      "classificationReason": "implementation_signal"
+    }
+  },
   {
     "name": "ambiguous_implementation_approval",
     "input": "Sounds good, go ahead.",