enhance(#2229): add three-way claim disposition (admit/refute/abstain) to /gsd-explore research pass (#2543)

Closes #2229.

Each claim surfaced by /gsd-explore's research pass is dispositioned admit, refute, or abstain, with abstentions routed to a visible ledger instead of being smoothed into confident prose. Refute and abstain are separated by whether the disagreeing source is authoritative for that claim; a strong prior is never authoritative alone.

Two guards ride with it: conflict-abstention, and a tier floor that presents a would-be admit as an abstain when the researcher's resolved tier is the budget tier or cannot be determined.

To make that floor enforceable, resolve-model now emits the effective tier (--pick tier). It was already computed above the resolve_model_ids omit gate but was unreachable from a workflow, which left the floor inert on every non-Claude install - the model id is blank under omit and runtime-substituted where a tier map exists, and the profile defaults to balanced. The tier signal mirrors every resolution step that can change which tier runs, including the model_policy preset, and reports unknown rather than guessing. Output is additive; model, profile and effort are unchanged.

Two residuals are disclosed in the workflow rather than papered over: a raw-model-id model_overrides pin reports unknown and is floored (fails closed), and a model_profile_overrides entry repointing a tier at another tier's model can under-report (fails open, and predates this change).

Admin merge used only to satisfy the missing secondary reviewer on a single-maintainer PR. No CI failure and no conflict were bypassed: 38 checks green, remote runner 32255/32255 on both Node lanes.

docs/FEATURES.md

@@ -2222,6 +2222,7 @@ Test suite that scans all agent, workflow, and command files for embedded inject
 - REQ-EXPLORE-02: Session MUST offer to route outputs to the appropriate GSD artifact
 - REQ-EXPLORE-03: An optional topic argument MUST prime the first question
 - REQ-EXPLORE-04: Exploration MUST optionally spawn a research agent for technical feasibility
+- REQ-EXPLORE-05: A research pass MUST disposition each surfaced claim (admit / refute / abstain) and route every abstention to a visible Unresolved Ledger — never smoothing an ungrounded claim into the narrative as confident prose
 
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