Rename the three Stage 2/3 soil-moisture files to the canonical
4-part convention stage<N>_<param>_<kernel>_<observable>.py:

  stage2_parameter_fitting.py     -> stage2_sm_AVS_hh.py
  stage3_soil_moisture_retrieval.py -> stage3_sm_AVS_hh.py
  stage23_fit_and_retrieve.py     -> stage23_sm_AVS_hh.py

The new pattern encodes parameter (sm), kernel (AVS), and primary
observable (sigma0_HH) in the filename. Stage 2/3 names are
symmetric: SM is no longer privileged as the unnamed default,
which sets the shape for future ivf/vod/swe kernels.

Updates all references in active code (dps/*.sh, skills/maap_exec.py,
src/algorithm/fitting.py docstring) and active docs (README, docs/index,
docs/atbd/stage3_aprs, docs/implementation/{algorithm_entry_points,
io_to_code,module_reference,orchestration}, docs/claude_code_demo,
maap/S3_STRUCTURE_AND_WORKFLOWS, experiments/README). Cross-imports
inside stage23_sm_AVS_hh.py updated to the new module names.

PIPELINE.md v1 added at the repo root as the canonical statement of
the 4-stage taxonomy (0 grid / 1 prep / 2 forward modeling /
3 retrieval) and the code-to-stage mapping; the renamed files appear
in their new names there.

Historical references in docs/log.md, summary/handoffs, summary/sessions,
summary/lessons, ADRs, archive/, _backup-*, .claude/memory, and frozen
experiment PRDs are deliberately left at the old names as point-in-time
records.

Follow-up required (not in this commit): DPS algorithms registered
against the three renamed files (parameter-fitting, soil-moisture-
retrieval, fit-and-retrieve) need re-registration. Per
feedback_dps_build_wait memory, wait 15 minutes after re-registering
before submitting jobs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>