fix(wpcarro/ynab): Remove .skip-subtree

**TL;DR:**
- Delete half-baked packaging attempts (`job.nix`, `token.nix`).
- Ensure golang code compiles.
  - Some "packages" were being treated like "programs" presumably for
    debugging/testing purposes back when I was working on this. Make those
    behave like libraries.
  - Remove stale imports.
  - Fix syntax errors.
  - Miscellaneous other chores.
- Drop `shell.nix` and `use_nix` directive.

Change-Id: I63c275680bac55a3cad3b9cb48d51cdc431fbe48
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7318
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

users/wpcarro/tools/monzo_ynab/monzo/client.go

@@ -27,8 +27,8 @@ func Create() *Client {
 }
 
 // Returns a slice of transactions from the last 24 hours.
-func (c *Client) Transactions24Hours() []monzoSerde.Transaction {
-	token := tokens.AccessToken()
+func (c *Client) TransactionsLast24Hours() []monzoSerde.Transaction {
+	token := tokens.GetState().AccessToken
 	form := url.Values{"account_id": {accountID}}
 	client := http.Client{}
 	req, _ := http.NewRequest("POST", "https://api.monzo.com/transactions",