docs(plans): parametrised recipes + rename-preview plan

Drafts and grills the next M-L roadmap item before implementation:
parametrised recipes infra, diff/diff-json formatters, and the
rename-preview recipe.

Captured 9 pre-locked moat/floor decisions and resolved 12 design
questions up front, including params syntax, type coercion, validation
errors, frontmatter shape, diff output shape, stale-file handling,
alias-chain depth, narrowing params, string/comment caveats, MCP params
shape, recipe-layer stale handling, and required tests.

The plan ships as three tracer-bullet slices:
1. parametrised recipe infra + exemplar recipe
2. diff / diff-json output formatters
3. rename-preview bundled recipe

Also updates the docs index so the in-flight plan is discoverable.

Author: SutuSebastian

docs/README.md

@@ -21,7 +21,7 @@ Each topic has exactly one canonical file. Other files cross-reference by relati
 | [fixtures/qa/](../fixtures/qa/)               | [prompts.external.template.md](../fixtures/qa/prompts.external.template.md) — optional chat QA prompts for an external index (`*.local.md` gitignored).                                                                                                                                                                                                                                             |
 | [packaging.md](./packaging.md)                | **`CHANGELOG.md` / `dist/` / `templates/`** on npm, **engines**, [**Node vs Bun**](./packaging.md#node-vs-bun), [**Releases**](./packaging.md#releases) (Changesets; **`bun run version`** + oxfmt **`CHANGELOG.md`**).                                                                                                                                                                             |
 | [roadmap.md](./roadmap.md)                    | Forward-looking [**Backlog**](./roadmap.md#backlog) and [**Non-goals**](./roadmap.md#non-goals-v1) (not a `src/` inventory).                                                                                                                                                                                                                                                                        |
-| [plans/](./plans/)                            | One `<feature-name>.md` per in-flight plan. Created on demand — don't add the `-plan` suffix; the folder provides context. In flight: [`c9-plugin-layer.md`](./plans/c9-plugin-layer.md), [`lsp-diagnostic-push.md`](./plans/lsp-diagnostic-push.md).                                                                                                                                               |
+| [plans/](./plans/)                            | One `<feature-name>.md` per in-flight plan. Created on demand — don't add the `-plan` suffix; the folder provides context. In flight: [`c9-plugin-layer.md`](./plans/c9-plugin-layer.md), [`lsp-diagnostic-push.md`](./plans/lsp-diagnostic-push.md), [`parametrised-recipes-and-rename-preview.md`](./plans/parametrised-recipes-and-rename-preview.md).                                           |
 | [research/](./research/)                      | Dated, snapshot-style notes (e.g. competitive scans). Each note links shipped items back to canonical homes — see [research/competitive-scan-2026-04.md](./research/competitive-scan-2026-04.md).                                                                                                                                                                                                   |
 
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