docs: add docs/plans convention doc (#Phase2 WI-1)

docs/plans/README.md

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+# docs/plans -- Plan Document Convention
+
+This folder contains per-issue vertical slice plans for primetimetank21/dev-setup. Plans are
+the pre-implementation spec; they capture scope decisions, design decisions, and acceptance
+criteria for a given issue before code is written.
+
+---
+
+## Front-Matter Schema
+
+Every plan doc must begin with a YAML front-matter block:
+
+```yaml
+---
+issue: <GitHub issue number, integer>
+title: "Short human-readable title"
+status: <see Status Vocabulary below>
+created: <ISO date, YYYY-MM-DD>
+updated: <ISO date, YYYY-MM-DD>
+---
+```
+
+No other fields are required. `supersedes: <issue>` is optional when a plan replaces
+a prior one.
+
+---
+
+## Status Vocabulary
+
+| Status      | Meaning                                          |
+|-------------|--------------------------------------------------|
+| draft       | In progress; not ready for implementation        |
+| ready       | Approved; ready to implement                     |
+| in-progress | Implementation underway (PR open)                |
+| implemented | Shipped to develop; doc kept for reference       |
+| superseded  | Replaced by another plan doc (note supersedes)   |
+
+---
+
+## File Naming
+
+```
+NNN-slug.md
+```
+
+Where NNN is the GitHub issue number (zero-padded to 3 digits) and slug is a short
+kebab-case descriptor. Example: 468-customizable-install.md
+
+---
+
+## Layout
+
+Flat files in docs/plans/. No per-feature subfolders unless a plan accumulates
+supplementary assets (diagrams, data tables) that cannot live inline. Threshold: >10 plans
+or attached binary assets.
+
+---
+
+## Review History
+
+Do NOT commit review-panel changelogs to plan docs. The HQ coordinator maintains
+review history externally. Plan docs contain only the living spec; revision history is
+captured in git log and front-matter `updated` field.
+
+---
+
+## What Belongs Here
+
+- Problem statement
+- Scope (IN / OUT)
+- Design decisions (with rationale)
+- Algorithm / data structures / pseudocode as needed
+- Test plan / acceptance criteria
+- Out-of-scope items (tracked issues)
+
+## What Does NOT Belong Here
+
+- Reviewer names, cast names, or review attributions
+- Version changelog blocks (vN Changes sections)
+- Process notes (merge decisions, admin notes, coordinator instructions)
+- Implementation progress updates (use the GitHub issue for that)