Add a "configure go-live env vars" skill to walk users through azd env set for M365 + Teams

## Problem

Going live today requires the user to manually run ~5 `azd env set` commands and hunt down:

- `MAILBOX_OWNER_EMAIL` – their own UPN
- `TEAMS_TEAM_ID` – from the channel link's `groupId=`
- `TEAMS_CHANNEL_ID` – the `19:...@thread.tacv2` segment of the channel link
- `TEAMS_MENTION_USER_ID` – their AAD object id (`az ad signed-in-user show`)
- `TEAMS_MENTION_NAME` – their display name

We have `infra/scripts/discover-teams-ids.{sh,ps1}` and a doc block in `README.md` §5, but the user still has to:

1. Know the script exists.
2. Paste a Teams channel URL from their browser.
3. Glue the outputs together.
4. Re-run `azd provision` / `azd up`.

This is the #1 friction point right after `azd up`.

## Proposal

Add a Copilot skill (under `.github/skills/` or wherever this repo registers skills — see `skills/`) named something like **`configure-go-live`** that:

1. Detects current `azd env get-values` and reports which of the 5 vars are unset / placeholder.
2. Prompts the user for a Teams channel URL and extracts `TEAMS_TEAM_ID` + `TEAMS_CHANNEL_ID` via regex.
3. Runs `az ad signed-in-user show` to auto-fill `TEAMS_MENTION_USER_ID` + `TEAMS_MENTION_NAME` (and offers `MAILBOX_OWNER_EMAIL` from `userPrincipalName`).
4. Shows the diff (old → new) and asks for confirmation before issuing the `azd env set` calls.
5. Offers to run `azd provision` (or `azd up`) afterward, and reminds the user to run `./infra/scripts/authorize-connectors.sh` the first time.

## Acceptance

- A user with a fresh clone + `azd up` already run can go from DRY RUN → LIVE in a single skill invocation.
- Skill is idempotent (re-running it just re-confirms / updates whatever's stale).
- Works on both bash and PowerShell hosts.

## Related

- `README.md` §5 (Go live)
- `docs/configuration.md` (Go live with real M365)
- `infra/scripts/discover-teams-ids.{sh,ps1}`
- `skills/teams-channels.md`

Start by inspecting the existing skill registration under skills/, then read README.md §5, docs/configuration.md, skills/teams-channels.md, and infra/scripts/discover-teams-ids.{sh,ps1}. Run azd env get-values and review the existing discovery scripts before defining the skill flow. Done means one invocation supports dry-run-to-live configuration, confirmation and idempotent updates, works in bash and PowerShell, and offers the documented follow-up commands.

Written by the indexing model from the issue text.