docs: fix agents field type in plugin manifest reference

[clowerweb]

Summary

• Update agents field type from "String or Array of strings" to "Array of strings" in manifest-reference.md
• Fix the "Complete Plugin" example to use array syntax: ["./specialized-agents"]
• Add warning note that string values cause silent installation failure

The agents field in plugin.json only accepts an array of strings. Using the string form (e.g. "./agents/") causes plugin installation to silently fail with no error surfaced to the user. Multiple community reports confirm only the array format works.

Test plan

• Verify manifest-reference.md shows agents type as "Array of strings"
• Verify all agents examples in the repo use array syntax
• Confirm no other docs files reference the string form for agents

Fixes #21598

🤖 Generated with Claude Code

[ghassans-stripe]

Must be a regression - a string, without an array, works on my v2.1.19. The array also works.

[clowerweb]

Must be a regression - a string, without an array, works on my v2.1.19. The array also works.

Thanks for the feedback! The original issue reporter (#21598) found that string format caused silent installation failure when installing through the marketplace, though they didn't specify a version. It's possible string support worked in earlier versions and regressed. Either way, the array format works consistently across versions, so updating the docs to recommend that seems like the safer path. If string support is intentional and was accidentally broken, that's probably a separate bug fix in the core app.

[daleh-daemon]

The validation of either still fails