Show a colour-coded team picker instead of cycling with T

T used to silently cycle Model.activeTeamID through the caller's teams with
no visible list of choices. It now opens a full picker (modelled on the
existing user picker) listing every team plus "All teams", each with a
stable identity colour from a new six-colour theme palette. The same colour
now also shows as a bullet next to "team: <name>" in the header, so the
active team stays visible without opening the picker.

Adds an Identity palette to the theme package (six hues, skipping the ones
that already mean firing/critical), Styles.TeamColor to pick one by team id,
and identity_1..6 as theme-file tokens alongside the existing twelve so a
fully custom theme can still set every token.

README.md

@@ -131,7 +131,7 @@ accent: "#fabd2f"
 A file may shadow a built-in name — `themes/gruvbox-dark.yaml` is how you tweak
 the default without renaming it.
 
-Without `extends`, every token must be set. The twelve are:
+Without `extends`, every token must be set. The eighteen are:
 
 | Token | Where it shows |
 |---|---|
@@ -144,6 +144,7 @@ Without `extends`, every token must be set. The twelve are:
 | `resolved` | resolved alerts and incidents, the top-alerts chart |
 | `error` | error banners |
 | `sev_critical`, `sev_error`, `sev_warning`, `sev_info` | the `severity` label |
+| `identity_1` .. `identity_6` | a team's colour in the header and the `T` picker — carries no meaning of its own, so pick six colours that just read as distinct from one another (and from `firing`/`sev_critical`, which already mean something) |
 
 Values are hex (`#83a598` or `#abc`) or an ANSI palette index (`0`–`255`) if you
 would rather follow your terminal's own colours. Colours are downsampled