feat: add category and effect as first-class fields in extension schema (#2899)

* feat: add category and effect as first-class fields in extension schema

Add `category` and `effect` as optional fields in the extension schema
(`extension.yml`) and community catalog (`catalog.community.json`).

Schema changes:
- Valid categories: docs, code, process, integration, visibility
- Valid effects: read-only, read-write
- Both fields are optional (backward-compatible with existing extensions)
- Validation raises ValidationError for invalid values when present

Propagation:
- Added `category` and `effect` to all 108 entries in catalog.community.json
  (populated from the existing docs/community/extensions.md table)
- Updated extension template with commented category/effect fields
- Updated add-community-extension skill with new JSON template fields
- Updated `specify extension info` CLI output to display category/effect
- Added properties to ExtensionManifest class

Tests:
- test_valid_category: all 5 category values pass
- test_valid_effect: both effect values pass
- test_invalid_category: invalid value raises ValidationError
- test_invalid_effect: invalid value raises ValidationError
- test_category_and_effect_optional: omitting fields still works

Closes #2874

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: make category free-form, keep effect validated

Category is a free-form string (only validated as non-empty when present),
while effect remains restricted to 'read-only' or 'read-write'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: address PR review feedback

- Add type guard before 'in' check for effect to prevent TypeError on
  unhashable YAML values (list/dict)
- Comment out category/effect in template so authors must opt in
- Use VALID_EFFECTS constant in test instead of hard-coded values

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: update category docstring to reflect free-form semantics

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* docs: clarify canonical extension effect values

---------

Co-authored-by: Manfred Riem <mnriem@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

docs/community/extensions.md

@@ -7,7 +7,7 @@
 
 The following community-contributed extensions are available in [`catalog.community.json`](https://github.com/github/spec-kit/blob/main/extensions/catalog.community.json):
 
-**Categories:**
+**Categories** (common values, but any string is allowed):
 
 - `docs` — reads, validates, or generates spec artifacts
 - `code` — reviews, validates, or modifies source code
@@ -15,10 +15,13 @@ The following community-contributed extensions are available in [`catalog.commun
 - `integration` — syncs with external platforms
 - `visibility` — reports on project health or progress
 
-**Effect:**
+**Effect** (canonical `extension.yml`/catalog values):
 
-- `Read-only` — produces reports without modifying files
-- `Read+Write` — modifies files, creates artifacts, or updates specs
+- `read-only` — produces reports without modifying files (displayed as `Read-only` in the table)
+- `read-write` — modifies files, creates artifacts, or updates specs (displayed as `Read+Write` in the table)
+
+> [!TIP]
+> Extension authors can declare `category` and `effect` in their `extension.yml` under the `extension:` block. These fields are also available in `catalog.community.json` for tooling and the CLI (`specify extension info`).
 
 | Extension | Purpose | Category | Effect | URL |
 |-----------|---------|----------|--------|-----|