docs(station): Backlog P2 — nonint.validateOverlay placeholder refactor

Filed during Plan 39 code review. `internal/nonint/config.go:120`'s
project_name placeholder hack works but obscures intent. Cleaner approach:
extend config.Validate with SkipRequiredFields option. ≤30 LOC.

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

Author: LastStep

station/Playbook/Backlog.md

@@ -84,6 +84,7 @@ Items that should be worked together are tagged with a group letter. See the gro
 - **[improvement] OSS polish — demo GIF/asciinema for README** — Last remaining OSS readiness item after Plan 17. Linter config (`.golangci.yml` with errcheck/govet/unused/misspell/gofmt/goimports) and Makefile `test`/`lint`/`fmt`/`tidy` targets shipped in PR #24. Still need a demo GIF or asciinema recording to add under README hero image — requires user recording (not agent-able). When recording: show `bonsai init` flow, add a skill, run `bonsai list`. *(added 2026-04-16, narrowed 2026-04-17 post-Plan-17, source: RESEARCH-oss-readiness.md cleanup)*
 - **[improvement] Ability-name argument completion for `bonsai add <skill>`** — `bonsai completion` (PR #78, #54) generates static command/flag completions but ability-name args still tab-dead-end. Wire dynamic `ValidArgsFunction` for `add`/`remove`/`update` that reads installed/available abilities from catalog. Out-of-scope hint in #54. *(added 2026-05-07, source: PR #78 follow-up)*
 - **[debt] Plan 26 candidate — skills frontmatter convention decision** — 13 of 17 skills lack YAML frontmatter (only `dispatch`, `issue-classification`, `pr-creation`, `workspace-guide` have it). Catalog loader reads triggers from `meta.yaml` not file frontmatter, so this is cosmetic — but a consistent convention should be picked: (a) require frontmatter on all skills, (b) drop frontmatter from the 4 that have it, (c) document that frontmatter is optional and skip-list specific use cases. Once decided, mass-apply across catalog. *(added 2026-04-22, source: v0.2.0 audit P2 deferred)*
+- **[debt] `nonint.validateOverlay` placeholder hack — refactor `config.Validate` to take skip-required flag** — `internal/nonint/config.go:120` sets `c.ProjectName = "_overlay_placeholder_"` + defer-restores to bypass `config.Validate`'s project-name-required rule for overlay configs. Works, but obscures intent. Cleaner: extend `config.Validate(opts ...ValidateOption)` with a `SkipRequiredFields` option (or `ValidateOverlay()` companion method). Touch ≤30 LOC. *(added 2026-05-13, source: Plan 39 code review)*
 
 ### Group D: Catalog Expansion