Document Patch tool input hooks and chat channel filter context

[mhornbacher]

Documents two additions to tool call hooks for their launch.

Stacked on #432 — that PR rewrites tool-call-hooks.mdx (new pre-output event, filter-variables-by-event table, eleven built-in patterns), so this targets its branch rather than main to avoid conflicting edits. Retarget to main once #432 merges.

What changed

product/admin/tool-call-hooks.mdx only.

New hook type — ## Patch tool input hooks, placed between the built-in patterns and custom function sections, since it sits between them in complexity. Follows the depth of the neighboring type sections: what it's for, the pre-tool-use-only constraint, the two configuration modes (CEL expression and static JSON), a ### Patch expression variables table (ctx, input, caller), ### How the patch is applied, and two worked examples — stamping a scope onto Datadog calls from one Slack channel, and a static patch that pins one field and drops another.

The merge behavior is explained in plain terms first — a key is added, overwritten, or removed when set to null, and unmentioned keys are untouched — with a link to JSON Merge Patch for precision rather than as the explanation. This matches how the docs cite specs elsewhere (for example step-up auth linking RFC 9470).

New filter context — ### Chat channel context under Configure a hook, documenting ctx.surface and ctx.channel_id on the pre-tool-use and post-tool-use events. Emphasizes that both are absent rather than empty for calls that didn't come from a chat channel, so filters need has() first — otherwise the expression fails to evaluate and the fail-closed rule denies the call. The existing Filter variables by event table points here, and the context block in the custom function payload gains both fields.

Also updated: the frontmatter description, the Hook type and Event rows of the configuration form table, and the hook-type step of the setup flow.

Test plan

• npx mintlify broken-links — no new broken links; the 10 reported are pre-existing in unrelated files. All new anchors verified against real headings.
• Review the rendered preview.

[mhornbacher]

Closing — this was mistakenly stacked on an unmerged branch (sathya/add-agent-guardrails-doc). Filters are GA, so this doc addition doesn't need to wait on that branch; redoing it directly against main in a new PR.