refactor: copilot project

[daabr]

Fixes in general:

• Use Copilot terminology from Copilot settings page: "cancel seat" instead of "un/subscribe" and "un/register" (except in the metadata description - where brevity is important)

Fixes in README.md:

• Removed ":" from a header
• Removed obsolete project var GITHUB_ORG
• Removed obsolete warning at the bottom, after https://github.com/autokitteh/docs.autokitteh.com/pull/143

Fixes in autokitteh.yaml:

• Removed obsolete project var GITHUB_ORG
• Renamed project vars and default Slack channel to be more self-describing
• IDLE_USAGE_THRESHOLD: changed debug values (10m) to stated default (72h)

Replaced helpers.py with users.py (adapted from new Purrr, like helpers.py)

Fixes in users.py compared to helpers.py:

• "Helpers" is too generic, always use specific names
• Never return None when a function declares that it returns only str
• When calling Slack functions, rely only in try & except SlackApiError, checking resp.get("ok", False) is never reached in case of an error (unlike our old Starlark wrapper)
• _slack_users function: Python rewrite doesn't need recursion (unlike our old Starlark wrapper)

Fixes in msg.json:

• UX principle: whenever possible, the "yes" option should be "first" (on the left), and the "no" option should be second (on the right)

Fixes in seats.py:

• Redundant constants and globals: if used only once, embedded them where they're used
• Use new project var names from the manifest (more descriptive)
• Lumped GitHub globals together
• prune_idle_seats function is unnecessary - embedded in find_idle_seats with a new input boolean
• t timestamp: renamed to be more self-describing (now), but more importantly eset for each seat instead of using a single initial value (the loop should be quick, but may not be)
• status line too line, split into 2 lines (readability is usually more important than efficiency)
• Reversed is_idle to is_active: results in flatter code at the bottom of the loop (if is_active: continue instead of if is_idle: ...)
• seat_dict: renamed, don't specify types in variable names
• seat_dict: changed type from dict[str, Any] to dict[str, str] - it's our object, no need to mimic GitHub's nested object structure
• Replaced "engage seat" (which doesn't mean a lot?) with "interact with user" in function name and comments
• Removed test-only comment and code
• Subscribe: use event_type instead of data.type
• Use more modern Python match-case instead of if-elif-else
• report function: handle gracefully empty SLACK_LOG_CHANNEL to enable making this project var optional

Note: we don't really need autokitteh.start, we could use the same trick as in the Slack sample project (embed important values e.g. GitHub/Slack user IDs in the interactive message and get them when the user clicks a button), but I guess there's educational value in showcasing autokitteh.start.

Fixes in triggers.py:

• on_schedule function: print each seat on its own instead of the whole list - more readable
• _get_logins function: renamed arg, don't specify types in variable names