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>

Author: mnriem

.github/skills/add-community-extension/SKILL.md

@@ -70,6 +70,8 @@ Use the existing entries as the format template. Required fields:
     "documentation": "<documentation>",
     "changelog": "<changelog>",
     "license": "<license>",
+    "category": "<category>",
+    "effect": "<effect>",
     "requires": {
       "speckit_version": "<speckit_version>"
     },
@@ -87,6 +89,9 @@ Use the existing entries as the format template. Required fields:
 }
 ```
 
+**Category** — one of: `docs`, `code`, `process`, `integration`, `visibility`
+**Effect** — one of: `read-only`, `read-write`
+
 If the extension has optional tool dependencies, add a `"tools"` array inside `"requires"`:
 
 ```json

docs/community/extensions.md

@@ -20,6 +20,9 @@ The following community-contributed extensions are available in [`catalog.commun
 - `Read-only` — produces reports without modifying files
 - `Read+Write` — modifies files, creates artifacts, or updates specs
 
+> [!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 |
 |-----------|---------|----------|--------|-----|
 | Agent Assign | Assign specialized Claude Code agents to spec-kit tasks for targeted execution | `process` | Read+Write | [spec-kit-agent-assign](https://github.com/xymelon/spec-kit-agent-assign) |