docs(helm): document supported values and add usage examples

[chihkang]

Summary

This PR improves the Helm chart documentation by adding several supported but currently undocumented values to the values reference and README.

Users can already rely on these options in practice, but today they are difficult to discover unless they inspect the chart templates directly. This change makes the chart easier to use and reduces trial-and-error during deployment.

Closes #163.

Changes

• add fullnameOverride to values.yaml
• add nameOverride to values.yaml
• document envFrom in values.yaml comments and chart docs
• document agentsMd usage with --set-file
• document the discord.allowedChannels --set-string requirement more prominently
• add a chart-level Helm values reference with practical examples
• link the main README Helm install section to the chart values reference

Why

The chart already supports these configuration paths, but they are not clearly documented. That creates unnecessary friction for users, especially for:

• multi-instance deployments
• secret-based environment injection
• large AGENTS.md configurations
• Discord channel ID configuration

Notes

This PR is documentation-focused and does not change chart behavior. It only makes existing supported options easier to discover and use.

Testing

• reviewed chart templates to confirm the documented values are already supported
• documentation change only; helm CLI was not available in the local environment for helm lint