docs: fix incorrect field naming restrictions in base-action/action.yml

Fixed outdated documentation that incorrectly stated field naming
restrictions that don't exist in the implementation.

Changes:
- Removed incorrect claim about field naming requirements (letter/underscore start)
- Removed incorrect claim about special character sanitization
- Clarified that field names can use any valid JSON property name
- Updated access pattern to show fromJSON() usage
- Clarified 1MB limit applies to entire structured_output string, not per-field

The implementation simply does JSON.stringify(result.structured_output)
without any sanitization, so any valid JSON property name works (including
hyphens like "test-result", as validated by integration tests).

Addresses review comment: https://github.com/anthropics/claude-code-action/pull/683#discussion_r2539749593

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

base-action/action.yml

@@ -74,14 +74,14 @@ inputs:
   json_schema:
     description: |
       JSON schema for structured output validation. Claude must return JSON matching this schema
-      or the action will fail. Outputs are automatically set for each field.
+      or the action will fail. All fields are returned in a single structured_output JSON string.
 
-      Access outputs via: steps.<step-id>.outputs.<field_name>
+      Access outputs via: fromJSON(steps.<step-id>.outputs.structured_output).<field_name>
 
       Limitations:
-      - Field names must start with letter or underscore (A-Z, a-z, _)
+      - All fields are returned in a single structured_output JSON string
-      - Special characters in field names are replaced with underscores
+      - Field names can use any valid JSON property name (including hyphens, special characters, etc.)
-      - Each output is limited to 1MB
+      - The entire structured_output string is limited to 1MB
     required: false
     default: ""