feat: negative message IDs as reverse inbox indices

[Copilot]

Commands accepting <message-id> required knowing the exact ID. Users now pass negative indices (-1, -2, …) to reference recent messages by position.

How it works

• -1 → most recent inbox message, -2 → second most recent, resolved via GET /fmsg (ID desc) with offset -N - 1
• fmsg get -1, fmsg del -1, fmsg get-data -2, etc. all work naturally

Changes

• cmd/msgid.go — resolveMessageID(client, arg): positive IDs pass through; negative values hit GET /fmsg?limit=1&offset=<N-1> to resolve the actual ID
• cmd/root.go — injectDashDash() prepended to Execute(): scans os.Args and inserts -- before the first -\d+ argument so pflag doesn't swallow it as a shorthand flag — users type fmsg get -1 with no special syntax required
• All 9 message-ID commands (get, del, update, add-to, attach, get-data, get-attach, rm-attach, draft send) updated to call resolveMessageID
• cmd/msgid_test.go — unit tests for resolution logic and injectDashDash (positive, -1/-2/-3, out-of-range, zero, invalid, flags-before-index)
• README.md — documents the feature with index table and examples

fmsg get -1            # most recent received message
fmsg get-data -2       # body of second most recent
fmsg update --topic "hello" -1   # flags must precede the negative ID