Add the catalog index, and package a real prompt collection

CANP-WP-0004. The operator asked canned-prompts to build a database of
versioned prompts recording where each came from and when — the first step
toward a platform for collaborative prompting.

The format had nowhere to put that. `provenance` records who wrote a prompt and
where the idea came from; nothing recorded how a copy arrived in a particular
store. Section 20.3 now specifies an `index.yaml` as store metadata rather than
package data: how a copy arrived differs for every consumer, and recording an
arrival must never rewrite the package that arrived.

`add`, `install` and `publish` record registry, id, version, name, source,
method, first-inclusion date, and the package's declared author, source and
licence — the last three copied so a listing is readable without opening every
package. A new `index` verb lists it. `included_at` is never overwritten; a
re-run updates `last_seen_at`, because when a package first entered a
collection is a fact about history rather than about the last command run.

Tests 84 -> 90.

Also records two findings from actually using the format:

CANP-WP-0004-T03 — inclusion has no deduplication, so a diamond dependency
renders shared content once per path. Found by composing a real collection.
Not fixed here: deduplicating means choosing which occurrence survives and
deciding what happens when two paths resolve different versions, which is
resolver behaviour that section 10.4 deliberately avoids. Handed to
CANP-WP-0005 with a leaning: document it, warn at validation time, do not
deduplicate.

The add_ons workaround in practice/pqrst-estimate is evidence about section
23's deferred "richer template syntax" — an optional appendix has to be an
input with an empty default, because CPF has no conditionals.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bjefh8NUiEiahN4JLwoSKM

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 388925@bnt-lap001
Assistant-Session: 3507023f-e0fd-4a1e-9d90-a0d4217d1502

README.md

@@ -181,6 +181,26 @@ signing and trust scoring are explicit non-goals. Ownership lives with the
 registry rather than in the package, so no package carries an unverifiable
 assertion of authority.
 
+## The index
+
+Every store keeps an `index.yaml` recording which package versions entered it,
+from where, and when:
+
+```bash
+python canned_prompts.py index
+```
+
+```text
+local:helix/repo-orient@0.1.0  2026-09-06T13:31:02Z  add
+                               from /home/worsch/helix-forge/prompts/repo-orient
+                               declares source personal prompt collection, contributed 2026-09-06
+```
+
+This is store metadata, not package data. `provenance` records who wrote a
+prompt; the index records how a copy arrived *here* — which differs for every
+consumer, and which must never rewrite the package it describes. `included_at`
+is first arrival and is never overwritten; a re-run updates `last_seen_at`.
+
 ## Dependencies are reported, not fetched
 
 Installing a package that composes another warns when the dependency is