docs(skills): add implement-batch skill for batch issue implementation

[s-annam]

Summary

Adds the implement-batch skill — a GitHub-only, self-contained batch driver for
okfmem. It implements an epic's sub-issues (or an explicit #,#,# list) onto
one feature branch as a single accumulated commit, running one isolated subagent
per issue (context-isolation is the mechanism, not an optimization), then a
bounded adversarial review pass, finalizing through /open-pr.

Adapted from the offlinecv sibling and customized to okfmem's facts.

No dedicated tracking issue — this is skill tooling. (Refs omitted.)

What changed vs the offlinecv original

• Gates → pure-Python stdlib. Per-issue: scripts/check-leaks.py (hard) +
  affected pytest + advisory ruff check. Dropped the TS/React
  typecheck/build/fallow steps; the full OS-matrix verify is the PR gate.
• Reviewer → ecc:python-reviewer (fallback ecc:code-reviewer, then a
  general-purpose subagent running /code-review).
• Non-negotiable rule → public-repo leak rule (replaces fixture-PII):
  no private strings in tracked files; check-leaks.py must exit 0.
• New blocking review classes specific to okfmem: confirmation-discipline
  rung (state-changing ops), and Windows/path invariants (pure-ASCII .ps1,
  path-by-shape, expected assertion values from the code's own normalizer).
• Finalize → okfmem /open-pr (leak-gate preflight + single-commit collapse
  for linear history); s-annam/okfmem protected main, per-issue-keyword
  auto-close.

Retains: subagent-per-issue isolation, blocked_by topological ordering,
threaded handoff notes, per-stage self-reported provenance, the single Phase-1
human confirmation gate, and the marker-guarded ## Adversarial review PR-body
append.

Test plan

• python3 scripts/check-leaks.py exits 0 (scanned 65 files, no leaks)
• ruff check . reviewed (advisory; no Python changed — docs-only)
• Skill file is well-formed markdown with valid frontmatter; discoverable as
  /implement-batch

Provenance

Skill authoring (adapt offlinecv → okfmem) via: Claude Opus 4.8 (1M context) — high effort
Verification: CI verify — pending