docs: expand sanitized outputs documentation and add deprecation notes for needs.activation.outputs.text/title/body

[Copilot]

Hippo repeatedly recalled deprecation guidance for needs.activation.outputs.{text,title,body}. No workflow .md files or docs actually used the deprecated form — all were already correct — but the reference docs only mentioned steps.sanitized.outputs.text, leaving title and body undocumented and the deprecation of the old form unmentioned.

Changes

• .github/aw/context.md: Expanded the sanitized outputs section to document all three outputs (text, title, body) with a per-event behavior table and an explicit deprecation blockquote clarifying that only these three are affected — other needs.activation.outputs.* fields (comment_id, slash_command, etc.) remain valid in downstream jobs.

• .github/aw/github-agentic-workflows.md and .github/aw/workflow-constraints.md: Updated the one-liner security rule to enumerate all three variants and explicitly warn against the deprecated aliases.

• docs/src/content/docs/reference/templating.md: Added a :::note[Deprecated form] callout after the sanitized outputs list.

Before (context.md):

- Use `${{ steps.sanitized.outputs.text }}` for untrusted user content.

After:

- Use `${{ steps.sanitized.outputs.text }}` (full context), `${{ steps.sanitized.outputs.title }}` (title only),
  or `${{ steps.sanitized.outputs.body }}` (body only) for untrusted user content.
  Do not use the deprecated `needs.activation.outputs.text/title/body` aliases.