docs: add Agent Action Capsule (capsule-emit) to the integrations catalog

[thisjody]

Adds an observability integration page for capsule-emit: ADKCapsuleEmitter attaches via ADK's standard before_tool_callback/after_tool_callback and seals each completed tool call as a content-addressed Agent Action Capsule — audit evidence whose integrity a third party can re-check offline (structure, content addressing, chain links), complementary to tracing.

Accuracy notes for reviewers:

• Every code block on the page was executed against the released PyPI wheel via a snippet gate (4 blocks: syntax, imports, bindings, execution; reference verifier returns OK on the produced ledger).
• The page states scope honestly: digests are commitments (not encryption), integrity is not completeness, and tamper-evidence is relative to a reference the other party holds — no overclaims.
• Catalog frontmatter follows the existing shelf pattern (catalog_* fields, SVG icon in assets/, observability tag); no nav changes needed (the shelf globs /integrations/*.md).

Disclosure: I co-maintain capsule-emit and am engaged by its sponsoring project (Action State Group); this page is submitted in that capacity. Drafted with AI assistance; reviewed, executed, and verified by a human before submission. Google Individual CLA signed under this account.

🤖 Generated with Claude Code

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[thisjody]

@googlebot I signed it!