`_register_skill_group` captured the skill catalog in closure variables
(`entries` and `skill_lookup`) so the single `tree.add_command` call at
startup owned the only live copy. The closure is never re-entered after
startup, so `/reload-skills` — which rescans the on-disk skills dir and
refreshes the in-process `_skill_commands` registry — had no way to
propagate results into the `/skill` autocomplete on Discord. New skills
stayed invisible in the dropdown, and deleted skills returned
"Unknown skill" when the stale autocomplete entry was clicked.
The fix is purely a dataflow change: promote `entries` and `skill_lookup`
to instance attributes (`_skill_entries`, `_skill_lookup`), split the
collector-driven rebuild into a helper (`_refresh_skill_catalog_state`),
and add a public `refresh_skill_group()` method that re-runs the helper
and is safe to call at any point after the initial registration.
The gateway's `_handle_reload_skills_command` then iterates
`self.adapters` and calls `refresh_skill_group()` on any adapter that
exposes it (currently only Discord). Both sync and async implementations
are supported; adapters that don't override the method (Telegram's
BotCommand menu, Slack subcommand map, etc.) are silently skipped — the
in-process `reload_skills()` call covers them.
No `tree.sync()` is required because Discord fetches autocomplete
options dynamically on every keystroke — mutating the instance state the
callbacks already read from is sufficient. That sidesteps the per-app
command-bucket rate limit (~5 writes / 20 s) that made the previous
bulk-sync-on-reload approach unusable (#16713 context).
Tests: tests/gateway/test_reload_skills_discord_resync.py — five cases
covering (1) refresh replaces entries, (2) entries stay sorted after
refresh, (3) collector exception leaves cached state intact, (4)
`_refresh_skill_catalog_state` populates the instance attrs, (5)
orchestrator calls `refresh_skill_group()` on sync + async adapters and
skips adapters that don't expose it.
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