[No QA] Docs updates for E/A#93751 Sentence-case agent rules naming and fix button label

[MelvinBot]

Why these docs changes

Expensify/App#93751 revised the in-product Agent rules copy in src/languages/*. Two changes affect the help site:

1. Capitalization → sentence case. The feature/section title is now Agent rules (was Agent Rules), and the buttons are Add agent rule / Find agent rule (was Add Agent rule / Find Agent rule).
2. Button label. The current "add" button is Add agent rule. The help article documented a non-existent Add AI Rule button.

Per docs/HELPSITE_NAMING_CONVENTIONS.md ("Use the exact text shown in the product UI", "Match capitalization exactly") and docs/HELP_AUTHORING_GUIDELINES.md ("Feature names match UI"), the articles should follow the updated UI.

What changed

Aligned the feature name to sentence case (Agent rules as the standalone label / sentence start, agent rule(s) as a common noun elsewhere) and corrected UI-element references across the four articles that mention the feature:

• docs/articles/new-expensify/ai-agents/Create-Agent-Rules.md — also corrects the documented button from Add AI Rule → Add agent rule and select the AI rule → select the agent rule.
• docs/articles/new-expensify/ai-agents/How-to-Write-Agent-Rules.md
• docs/articles/new-expensify/ai-agents/Create-and-Use-Custom-Agents.md — cross-references only.
• docs/articles/new-expensify/workspaces/Workspace-Rules.md — single cross-reference.

No structural, navigation, or workflow content was changed. Subtitle/describeRuleTitle strings from the PR are not quoted in the help site, so no copy was lifted from them.

Notes for the reviewer

• The new UI strings come from the merged PR diff, treated as the source of truth. The Agent rules feature is behind an open beta, so it could not be verified against a live UI session.
• The capitalization scope is a judgment call: I treated agent rules as a common noun to match the sentence-case UI. If the team prefers to keep Agent Rules as a branded proper noun in docs, this can be scoped down to just the button-label fix.

@MelvinBot, please review the files changed and confirm they reflect the current behavior. Then mark this PR Ready for review.

[OSBotify]

A preview of your ExpensifyHelp changes have been deployed to https://313c8fdb.helpdot.pages.dev ⚡️

Updated articles:

• articles/new-expensify/ai-agents/Create-Agent-Rules
• articles/new-expensify/ai-agents/Create-and-Use-Custom-Agents
• articles/new-expensify/ai-agents/How-to-Write-Agent-Rules
• articles/new-expensify/workspaces/Workspace-Rules

[MelvinBot]

🤖 Closing as a duplicate of #93869, which was created first and contains the identical changes. Consolidating the help site review for E/A#93751 into that single PR to avoid duplicate review.