[instructions] Sync instruction files with release v0.82.8

[github-actions]

Summary

Syncs instruction files with release v0.82.8. Updates two instruction documents in .github/aw/ to reflect new CLI commands and message configuration options introduced in this release.

Changes

.github/aw/cli-commands.md

• Adds documentation for the new gh aw experiments command, including experiments list and experiments analyze subcommands with --repo/-r and --json/-j flags.
• Notes that gh aw experiments has no MCP equivalent and must be run from a local terminal.
• Adds gh aw experiments row (marked *(local only)*) to the CLI-to-workflow mapping table.

.github/aw/messages.md

• Adds a new Body headers subsection documenting two new message keys:
  • disclosure-header — AI-authorship disclosure prepended to every AI-generated body; off by default; accepts true for built-in text or a custom string.
  • body-header — custom header text prepended to every body; off by default.
• Both keys accept {workflow_name} and {run_url} template variables.
• Documents insertion order: threat-detection caution → disclosure-header → body-header → agent content.
• Applies to issues, comments, pull requests, and discussions.

Type

docs — instruction/reference documentation update only; no source code changes.

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