docs: reconcile the two parallel implementation-plan locations (docs/plans vs docs/implementation-plans) and fix stale path references left by the plan-removal cleanup

[bpowers]

Problem

The recent docs cleanup commit 287ec271 ("docs: remove old implementation plans") deleted all docs/implementation-plans/<plan>/ subdirectories but left the docs in a half-finished restructure with two distinct, non-overlapping problems plus a structural inconsistency. Commit 2ed93950 already fixed the broken-link fallout (de-linked dead markdown links so the documentation-link pre-commit check passes), so this is not gate-blocking -- but stale prose/code-span references and a confusing two-location structure remain.

1. Stale prose / code-span references to deleted docs/implementation-plans/... paths

These are inline-code references (not markdown links, so the link checker doesn't catch them) that point a reader at directories that no longer exist:

• docs/test-plans/2026-05-11-ltm-arrays-hardening.md:3 -- "verifies the implementation in `docs/implementation-plans/2026-05-11-ltm-arrays-hardening/`"
• docs/test-plans/2026-05-11-ltm-arrays-hardening.md:5 -- "see ... `docs/implementation-plans/2026-05-11-ltm-arrays-hardening/test-requirements.md`"
• docs/test-plans/2026-05-13-macros.md:3 -- "Companion to the implementation plan at `docs/implementation-plans/2026-05-13-macros/`"
• docs/test-plans/2026-05-09-ltm-503-cross-element-agg.md:5 -- "see ... `docs/implementation-plans/2026-05-09-ltm-503-cross-element-agg/test-requirements.md`" (not in the original report, found during triage)
• docs/test-plans/2026-04-05-server-rewrite.md:3 -- "implemented under `docs/implementation-plans/2026-04-05-server-rewrite/`"
• docs/tech-debt.md -- multiple lines reference `docs/implementation-plans/` as the canonical workflow location: line 579 (item 61 suggested fix), 585 (item 62 Component), 594/596/597 (item 63 Component/Description/Suggested fix).
• docs/README.md:34 -- the documentation index still carries an `implementation-plans/` heading ("Detailed phase-by-phase implementation plans, created during plan execution") describing a directory that is now empty/untracked. (not in the original report, found during triage)
• docs/design-plans/2026-05-11-ltm-arrays-hardening.md:395 -- prose reference to the `writing-implementation-plans` flow / plan dir.

2. Structural: two parallel plan-location structures, and the workflow uses the untracked one

• docs/implementation-plans/ exists on disk as an empty, untracked directory. `git ls-files docs/implementation-plans/` returns nothing, so on a fresh clone it will not exist at all -- any future link or reference to it is fragile.
• docs/plans/ is tracked (`docs/plans/README.md`, `docs/plans/active/.gitkeep`, `docs/plans/completed/.gitkeep`) and uses a different template (`# Plan: [Title]` with Status/Created/Owner/Goal/Context/Approach/Open Questions/Outcome sections).
• The repo now documents two implementation-plan locations. Critically, the actual workflow skills disagree with the tracked structure: `writing-implementation-plans` / `executing-an-implementation-plan` (and the plan-and-execute plugin README) create plans under `docs/implementation-plans/YYYY-MM-DD-/` (one file per phase plus `test-requirements.md`), not under the tracked `docs/plans/active/`. So the tracked `docs/plans/` structure is effectively orphaned, and the location the workflow actually writes to is the untracked-on-fresh-clone one.

This looks like an incomplete reorganization.

Why it matters

• Correctness / accuracy: readers (human or agent) following the stale `docs/implementation-plans/...` code-spans land on a directory that doesn't exist; the implementation evidence those test-plans/tech-debt entries point at was deleted.
• Developer experience / discoverability: `docs/README.md` is the canonical entry point and still advertises a directory that's empty/untracked.
• Process robustness: the plan location the workflow writes to (`docs/implementation-plans/`) is not tracked on a fresh checkout, while the tracked location (`docs/plans/`) is not what the workflow uses. Future plan work will keep recreating the fragile state.

Components affected

• docs/README.md (index heading)
• docs/test-plans/ (four files: 2026-04-05-server-rewrite, 2026-05-09-ltm-503-cross-element-agg, 2026-05-11-ltm-arrays-hardening, 2026-05-13-macros)
• docs/design-plans/2026-05-11-ltm-arrays-hardening.md
• docs/tech-debt.md (items 61, 62, 63 component/description/fix lines)
• docs/plans/ (tracked, orphaned structure + README template)
• docs/implementation-plans/ (empty, untracked directory)
• The writing-implementation-plans / executing-an-implementation-plan workflow skills (they hardcode docs/implementation-plans/)

Possible approaches

1. Pick one canonical location. Either:
   • (a) Adopt docs/implementation-plans/ as canonical: delete the orphaned docs/plans/ tree, and make docs/implementation-plans/ survive a fresh clone (add a .gitkeep or a tracked README.md so the dir + any docs/README.md link is robust). This aligns with what the workflow skills already do, so it requires the least skill churn.
   • (b) Adopt docs/plans/ as canonical: update the writing-implementation-plans / executing-an-implementation-plan skills (and the plan-and-execute README) to write under docs/plans/active/, and remove the empty docs/implementation-plans/ dir.
2. Update all references to whichever location is chosen: fix the stale prose/code-spans in the four docs/test-plans/ files, docs/design-plans/2026-05-11-ltm-arrays-hardening.md, docs/README.md:34, and the docs/tech-debt.md items 61/62/63 (or rephrase them to be location-agnostic, since the referenced plan content was deleted anyway).
3. Ensure the chosen directory is either git-tracked (so it exists on fresh clone) or only ever referenced via robust links.

Context

Discovered while writing a design plan; out of scope for that task, filed for tracking. The broken-link half was already fixed in commit 2ed93950 ("docs: add C-LEARN residual design plan; drop dead impl-plan links"). This issue covers the remaining stale prose/code-span references and the unreconciled two-location structure.

Related but distinct from existing tracked items: #577 (a single missing README index entry for one now-deleted plan dir -- index hygiene, not the reorg), #407 (pruning stale verbatim code from docs/design-plans/ -- different directory), and tech-debt #61/#62/#63 (CLAUDE.md decay / corpus-validation back-loading / plan-dir git-tracking timing). None of those addresses the post-deletion stale references or the docs/plans vs docs/implementation-plans two-location reconciliation.