refactor: centralize slash command registry (#1603)

* refactor: centralize slash command registry

Replace 7+ scattered command definition sites with a single
CommandDef registry in hermes_cli/commands.py. All downstream
consumers now derive from this registry:

- CLI process_command() resolves aliases via resolve_command()
- Gateway _known_commands uses GATEWAY_KNOWN_COMMANDS frozenset
- Gateway help text generated by gateway_help_lines()
- Telegram BotCommands generated by telegram_bot_commands()
- Slack subcommand map generated by slack_subcommand_map()

Adding a command or alias is now a one-line change to
COMMAND_REGISTRY instead of touching 6+ files.

Bugfixes included:
- Telegram now registers /rollback, /background (were missing)
- Slack now has /voice, /update, /reload-mcp (were missing)
- Gateway duplicate 'reasoning' dispatch (dead code) removed
- Gateway help text can no longer drift from CLI help

Backwards-compatible: COMMANDS and COMMANDS_BY_CATEGORY dicts are
rebuilt from the registry, so existing imports work unchanged.

* docs: update developer docs for centralized command registry

Update AGENTS.md with full 'Slash Command Registry' and 'Adding a
Slash Command' sections covering CommandDef fields, registry helpers,
and the one-line alias workflow.

Also update:
- CONTRIBUTING.md: commands.py description
- website/docs/reference/slash-commands.md: reference central registry
- docs/plans/centralize-command-registry.md: mark COMPLETED
- plans/checkpoint-rollback.md: reference new pattern
- hermes-agent-dev skill: architecture table

* chore: remove stale plan docs

tests/test_cli_prefix_matching.py

@@ -35,7 +35,9 @@ class TestSlashCommandPrefixMatching:
                 raise RecursionError("process_command called too many times")
             return original(self_inner, cmd)
 
-        with patch.object(type(cli_obj), 'process_command', counting_process_command):
+        # Mock show_config since the test is about recursion, not config display
+        with patch.object(type(cli_obj), 'process_command', counting_process_command), \
+             patch.object(cli_obj, 'show_config'):
             try:
                 cli_obj.process_command("/con set key value")
             except RecursionError:
@@ -57,7 +59,9 @@ class TestSlashCommandPrefixMatching:
                 raise RecursionError("Infinite recursion detected")
             return original_pc(self_inner, cmd)
 
-        with patch.object(HermesCLI, 'process_command', guarded):
+        # Mock show_config since the test is about recursion, not config display
+        with patch.object(HermesCLI, 'process_command', guarded), \
+             patch.object(cli_obj, 'show_config'):
             try:
                 cli_obj.process_command("/config set key value")
             except RecursionError: