Introduce feature flag system

[lukecarr]

Adds a feature-flag system so operators can toggle parts of Ember on or off at startup.

What

• Flag + FlagManager (api): catalog-style declarations with enabledByDefault, resolved via enabledByDefault XOR isSet.
• Flags catalog + SimpleFlagManager (plugin): single source of truth for declared flags; in-memory manager loaded once from plugins/Ember/flags.txt (lowercased + trimmed, with # comment support).
• Subcommand abstraction: subcommands can declare a gating flag; the subcommand builder extension skips registration when the flag resolves disabled, so gated features genuinely vanish from tab-completion.
• /ember flags list / /ember flags get <flag>: read-only inspection of the catalog with hover-rendered descriptions. Gated by the new ember.flags permission.
• Msg: Caffeine-backed MiniMessage cache (static / keyed / uncached-with-resolvers) so repeated renders skip retokenization.
• command.version is the first declared flag, gating /ember version.

Docs

• New server-owners/flags.mdx (conceptual + flags.txt format) and commands/flags.mdx (command reference).
• permissions.mdx gains ember.flags; configuration.mdx and the server-owners index link out to the flags page.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c941d2f) to head (2af6565).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.
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Additional details and impacted files

@@             Coverage Diff              @@
##                main        #9    +/-   ##
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  Coverage     100.00%   100.00%            
- Complexity        28        72    +44     
============================================
  Files              5        13     +8     
  Lines             58       169   +111     
  Branches          11        18     +7     
============================================
+ Hits              58       169   +111     

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Project	Status	Preview	Updated (UTC)
Ember	🟢 Ready	View Preview	May 19, 2026, 5:47 PM