Add read-path dry-run/explain mode: preview route + scope + deny without executing adapters

[se-jo-ma]

Context

Policy authors and operators want to preview a routing decision (sources_queried / sources_denied / sources_skipped / scope_restrictions / rule_trace) for a hypothetical request without querying any backend. Today the only way to see routing is to run a real request, which executes adapters and writes a normal audit entry.

Verified state of the code:

• A grep for dry_run|explain|preview|what.if across nautilus/ finds only simulate_meta_rule_fire (nautilus/rkm/__init__.py:31) and UI token_preview strings — nothing on the request path.
• The broker's public entrypoints request / arequest (nautilus/core/broker.py:988 / :1019) always run the full pipeline.
• _run_pipeline (nautilus/core/broker.py:1357) is a clean linear sequence: intent analysis → Fathom routing → scope merge → temporal filter → _build_adapter_jobs fan-out (line 1373) → synthesis → attestation → _update_session. A short-circuit before _build_adapter_jobs is well-defined.
• Precedent already exists: declare_handoff (nautilus/core/broker.py:1066) is documented as a "pure reasoning-only path (zero adapter calls, zero session-store mutation)" — the same shape this feature needs.
• BrokerResponse (nautilus/core/models.py:148) already carries sources_queried/denied/skipped and scope_restrictions; AuditEntry (nautilus/core/models.py:188) already carries rule_trace and a block of optional Phase-2 fields that round-trip unchanged (NFR-5) — so adding a dry_run marker follows the established optional-field pattern.
• The write-surface roadmap (design-docs/05-ecosystem-roadmap.md:513-515) already defines a dry_run: bool envelope audit flag for write rehearsals; this issue introduces the read-path analogue and should reuse the same audit-stamping convention. (This is distinct from the [P0] Write surface epic, which owns write rehearsal.)

Task

Add a dry-run path that runs intent analysis + Fathom routing + scope/temporal logic but skips _build_adapter_jobs / synthesis, returning routing + scope + denial + rule_trace with empty data.

• Add a dry_run flag to the broker public API. Note BrokerRequest (nautilus/core/models.py:31) is only the REST body model — Broker.request/arequest take positional args, so this also needs either a new keyword param on those methods or a dedicated Broker.explain(...) method. Pick one in design.
• Short-circuit _run_pipeline after _apply_temporal_filter (before line 1373) when dry-run is set.
• Cumulative-exposure / session state MUST NOT be mutated by a dry run (skip _update_session).
• Attestation: either skipped or clearly marked dry-run — never emit a signed token implying real data access.
• Stamp the audit entry with a dry_run marker (optional AuditEntry field, defaulting None/False to preserve NFR-5 round-trip).
• Surface via REST (a dry_run body flag on POST /v1/request, or a dedicated POST /v1/explain) and a new nautilus CLI subcommand (none of request/query/explain exists today — only version/health/serve are wired in nautilus/cli/__init__.py).

Where

• nautilus/core/broker.py — _run_pipeline (:1357), public request/arequest (:988/:1019); _build_adapter_jobs (:1565) is the boundary to skip
• nautilus/core/models.py — BrokerRequest (:31), BrokerResponse (:148), AuditEntry (:188)
• nautilus/transport/fastapi_app.py — /v1/request handler (~:274)
• nautilus/cli/__init__.py — subparser registration (~:68)

Acceptance criteria

• Dry-run returns sources_queried/sources_denied/sources_skipped/scope_restrictions/rule_trace with empty data and zero adapter calls (assert no adapter connect/query invoked).
• Cumulative-exposure / session-store state is NOT mutated by a dry run.
• The audit entry carries a dry_run marker distinguishing rehearsals from prod runs.
• Attestation is skipped or clearly marked dry-run; no signed token is issued that implies real access.
• Reachable via both REST and a nautilus CLI subcommand.
• uv run pytest -m unit passes; new unit tests cover the no-adapter / no-session-mutation invariants.

Size: L