[No QA] Update Custom Agent help docs

[MelvinBot]

Explanation of Change

Adds two new Custom Agent help articles under docs/articles/new-expensify/concierge-ai/:

• Custom-Agent-Triggers.md — Agent Trigger Reference: every report_activity event a Custom Agent can react to, with ready-to-copy instruction phrases.
• Custom-Agent-Capabilities.md — Agent Capability Reference: every action a Custom Agent can take, with ready-to-copy instruction phrases.

These are content updates for the Custom Agent help docs. This PR was opened on request as part of the ongoing work to fix the Custom Agent help-docs sync workflow (Web-Expensify PR #54175).

Fixed Issues

$
PROPOSAL:

Tests

// TODO: The human co-author must fill out the tests they ran before marking this PR as "ready for review".

This is a docs-only change ([No QA]). No app code is affected.

• Verify that no errors appear in the JS console

Offline tests

N/A — docs-only change.

QA Steps

// TODO: The human co-author must fill out any QA steps before marking this PR as "ready for review".

[No QA] — this change only adds Markdown help articles and does not affect the app.

• Verify that no errors appear in the JS console

[MelvinBot]

Fixed the failing spellcheck check and pushed to the branch.

Root cause: cspell flagged the word unsubmits/Unsubmits as unknown in two files:

• docs/articles/new-expensify/concierge-ai/Custom-Agent-Capabilities.md:57 (Unsubmits)
• docs/articles/new-expensify/concierge-ai/Custom-Agent-Triggers.md:70 (unsubmits)

Fix: Reworded both to the hyphenated un-submit form. cspell splits on hyphens (un + submit are both valid), and the same files already use un-submit/Un-submit elsewhere, which passes the check.

The other checks were passing or skipping; only spellcheck was failing. CI will re-run on the new commit.

[OSBotify]

A preview of your ExpensifyHelp changes have been deployed to https://68a7a826.helpdot.pages.dev ⚡️

Updated articles:

• articles/new-expensify/concierge-ai/Custom-Agent-Capabilities
• articles/new-expensify/concierge-ai/Custom-Agent-Triggers

[marcaaron]

Testing