fix(tmux): session env never reached the pane, so script agents ran blind - #279

…lind

Found by running a script-runtime agent from a column for the first time.

argv, cwd and exit-code advancement were already correct — the script got
exactly `--preset high --out cut.mp4`, with no prose prompt appended, and a
deliberately strict argument parser accepted it (`ARGC: 4`). But every
environment variable arrived unset:

    PRESET: <unset>          THREADS: <unset>
    TRIGGER_PROMPT: <unset>  WORKING_DIR: <unset>

Session environment was being applied with `Command::env` on the tmux
*client* process. The tmux server is a pre-existing daemon, so `new-session`
only asks it to create a session and the new shell inherits the **server's**
environment, not the client's. Everything set that way was silently dropped.

That is also why the `KAITENCODE_PARENT_*` attribution variables are
additionally inlined on the command line as `KEY=val <cmd>` — that
workaround covered exactly those and nothing else.

Consequences: a script agent's configured `env` was inert while the dossier
displayed it, and script agents received no prompt at all — neither in argv
(correctly, it would break strict parsers) nor in `$TRIGGER_PROMPT`. They
ran context-free. The claim in the previous commit that the prompt rides
`$TRIGGER_PROMPT` was true of the intent and false of the behaviour.

`tmux new-session -e KEY=VAL` sets it server-side and the pane created by
`new-session` inherits it, including values containing spaces and `=`.
Verified against tmux 3.4, which is what ships here. Applied to both session
creation paths — the trigger one and the interactive one, which had the same
bug.

Same run afterwards:

    PRESET: fast             THREADS: 8
    TRIGGER_PROMPT: Render the cut…  WORKING_DIR: …/worktrees/kaitencode-…

Also confirms the resolver's script handling from #275 holds in practice:
the column deliberately carried a stale `cli: claude` and `runtime_mode:
managed`, and both were correctly ignored — the run resolved to `terminal`
and executed the agent's own command.

One test-artifact worth recording: deleting a worktree out from under a live
tmux session leaves the pane's cwd on a removed inode, and the next command
fails with `getcwd: cannot access parent directories`. That is the situation
`column_is_terminal` already forces a fresh pane for; it was my reset
sequence, not a product bug.