Add root AGENTS.md (review-instructions pilot)

AGENTS.md

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+# AGENTS.md — cube_bathymetry
+
+Instructions for AI agents working in this repository — including **GitHub
+Copilot code review**, which reads this file when reviewing PRs. There is no
+`.agents/README.md` deep guide yet (the workspace convention for project
+agent guides — distinct from the `.agent/work-plans/` directory); start from
+the top-level `README.md` and `docs/` (including this repo's ADRs under
+`docs/decisions/`).
+
+## Workspace Rules
+
+This repo is developed inside a
+[ROS 2 Agent Workspace](https://github.com/rolker/ros2_agent_workspace).
+The workspace root `AGENTS.md` carries the full shared rules (worktree
+isolation, issue-first policy, commit conventions, AI signatures). This file
+**references** those rules and adds repo-specific context only — it must
+never restate or fork them.
+
+## Quality Standard
+
+This is software for autonomous robot boats operating on open water.
+Robustness is not optional.
+
+- Fix bugs completely: add the test, handle the edge case, check the
+  lifecycle transition.
+- Concerns about error handling, silent failures, stale data, or missing
+  validation are not nits — flag them unless the failure mode genuinely
+  cannot occur. "Config is under our control" and "pathological input" are
+  not blanket dismissals; field configs change under pressure.
+- A change includes its consequences: tests, documentation, and dependent
+  references update in the same PR.
+
+## Reviewing PRs
+
+- If the PR carries a work plan (`.agent/work-plans/issue-<N>/plan.md` or a
+  plan in the PR body), the plan is kept **in sync with the implementation
+  as it evolves** — an implementation that matches the current plan text is
+  not "plan drift", even if the plan changed after the PR opened.
+- Verify claims against source: parameters, topics, services, and message
+  types in docs must match the code.
+
+## Review Context — cube_bathymetry
+
+- **This implements CUBE** (Brian Calder's bathymetric estimation
+  algorithm): changes to estimator behavior need methodological
+  justification against the algorithm's published semantics, not just green
+  tests — algorithm fidelity is a review dimension of its own.
+- **Numerical/statistical code — watch signs and units**: depths vs
+  elevations, dB conventions, and transmission-loss corrections have all
+  produced sign bugs before; intensity statistics ride Welford accumulators
+  where order-independence and merge semantics matter.
+- **Eviction that bounds memory must never lose survey data**: draft-tile
+  eviction persists to the durable store *before* dropping from RAM; the
+  tile store is the product of record for live coverage.
+- **CI is industrial_ci with a prebaked GHCR deps image**:
+  `.github/workflows/ci.yml` (build/test), `.github/workflows/ci-image.yml`
+  (image publish), and `.github/ci/Dockerfile` must stay in sync —
+  dependency changes usually touch more than one.