feat(opencode): add /experimental slash command to enable experimental flags

[aravhawk]

Issue for this PR

Closes #7717

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a built-in /experimental slash command in the TUI so users can quickly enable experimental flags without manually editing env files.

Changes:

• Registers /experimental in built-in TUI commands.
• Adds an Experimental flags dialog that:
  • Lists available experimental env flags.
  • Supports boolean flags by setting KEY=true.
  • Supports numeric flags with a prompt and positive-integer validation.
  • Writes/updates values in project .env.
  • Shows success/error toasts and a restart reminder.
• Uses dynamic flag discovery from flag metadata so newly added experimental flags show up automatically (instead of a hardcoded list).
• Adds docs for /experimental in TUI docs.

Why this works:

• The command is integrated into the existing slash command and command palette system.
• The dialog reads flag definitions from the same flag layer used by runtime env handling, so UI options stay in sync with available experimental flags.
• .env update logic replaces existing lines (including export KEY=...) or appends when missing.

How did you verify your code works?

• Ran bun run --cwd packages/opencode typecheck (pass).
• Ran bun run --cwd packages/web build (pass).

Screenshots / recordings

1. Slash command discoverability (/experimental appears in autocomplete):

2. Experimental flags dialog:

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR