session background: per-pane target for split sessions (--pane left|right)

[DenisVanyushkin]

What

Let session background image|text|color|clear target one pane of a split session:

agtermctl session background color '#201414' --pane left  --target "$AGTERM_SESSION_ID"
agtermctl session background text  'PEER'    --pane right --target "$AGTERM_SESSION_ID"

Omitting --pane keeps today's behavior (the background applies to the whole session), so nothing
existing changes. Read-back would follow the same shape: e.g. background per pane in tree --json,
or a paneBackgrounds: {left, right} field next to the existing session-level one.

Why

A common two-agent setup puts one agent in the main pane and a second one in the split (for example
a Claude Code "driver" on the left and a Codex "peer" on the right, talking through session type /
session text --pane). Both panes run similar-looking TUIs, and at a glance it is easy to type into
the wrong one or misread which agent said what. A distinct tint or a faint watermark (DRIVER / PEER)
per pane would make the roles obvious at a glance.

Today session background is per session and applies to all of the session's panes, so both halves
always look the same. Several other commands already address panes with --pane left|right|scratch
(type, text, status), and overlay open --background-color / hud open --background-color already
color a single surface independently of the session, so pane-level addressing would fit the existing
model.

Workarounds considered

• OSC 11 written to the pane's tty works per surface, but it is lost when the agent restarts and it
  depends on the TUI not painting its own background. It also needs tty discovery outside agterm.
• A role label in each agent's own status line works, but it colors a single line, not the pane.
• session status --color only tints the sidebar glyph, and the tint is reset by the next status call.

Nice to have

• --pane scratch for parity with the other pane-addressed commands.
• The per-pane background survives session swap, following the pane's content rather than its position.

[umputun]

added in #647. session background image|text|color|clear takes --pane left|right|scratch:

agtermctl session background text DRIVER --opacity 0.12 --pane left --target "$AGTERM_SESSION_ID"
agtermctl session background text PEER --opacity 0.12 --pane right --target "$AGTERM_SESSION_ID"

a pane without an override inherits the session background, and clear --pane returns it to that. Left/right overrides survive a relaunch and follow their terminal across session swap and when the left pane closes. A scratch override ends when that scratch terminal closes. tree --json reports overrides as paneBackgrounds next to background.