Fix github add: catch channel_not_found, always DM caller, rename to 'add'

[ewels]

Summary

• Bug fix: chat.postMessage calls in the GitHub invite flow were failing with channel_not_found when the bot wasn't a member of the channel. The error bubbled to Bolt's generic handler, silently swallowing all feedback after "Looking up X on GitHub..." — even though the GitHub invitations themselves succeeded.
• DM failsafe: The caller now always receives a DM confirmation after a successful invite, so they're never left wondering whether it worked.
• Rename: Slash command shortened from /nf-core github add-member to /nf-core github add.

Changes

invite_flow.py

• invite_and_greet() now accepts a required client keyword arg and wraps every reply() call in _safe_reply() which catches exceptions and returns False
• New _reply_or_dm() helper deduplicates the reply-then-DM-fallback pattern (used at all 4 error sites)
• On reply failure for error messages, falls back to DMing the caller via _dm_caller()
• On success, always DMs the caller a confirmation ("Done! X has been invited...")

add_member.py

• _warn_missing_github() now catches chat.postMessage failures and falls back to ephemeral respond()
• Passes client=client to invite_and_greet() to enable DM failsafe

add_member_shortcut.py

• Wraps the missing-GitHub-username chat.postMessage in try/except with chat_postEphemeral fallback
• Passes client=client to invite_and_greet()

router.py

• Dispatch key changed from add-member to add

Docs & tests

• All references to add-member updated to add across README, CLAUDE.md, docs/, help text, and tests
• New test classes TestDmFailsafe covering success DMs and channel-reply-failure fallback
• Shared test helpers (make_slack_client, channel_messages, dm_messages) extracted to tests/helpers.py

Test plan

• All 360 tests pass
• ruff check clean
• mypy --strict clean
• Manual test: run /nf-core github add octocat and verify channel reply + DM
• Manual test: run shortcut from a channel the bot isn't in — verify DM fallback

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