Respect user tmux option overrides in status bar config

[johnuopini]

Summary

• When users define tmux options via [tmux] options that match agent-deck's managed status bar keys (status, status-style, status-left-length, status-right, status-right-length), agent-deck now skips its default for that key instead of silently overwriting it
• Extracted buildStatusBarArgs() from ConfigureStatusBar() to separate pure logic from subprocess execution, enabling unit testing
• Documented override behavior in the config template with a practical example (options = { "status" = "2" })

Motivation

Agent-deck's ConfigureStatusBar() hardcodes 5 tmux options per-session, applied on every session create/activate/focus change. Users who set these options via [tmux] options in config.toml had their values immediately overwritten — making it impossible to customize the status bar (e.g. multi-line status with status = "2") while keeping agent-deck's notification bar.

Approach

buildStatusBarArgs() builds the tmux command dynamically. For each of the 5 managed defaults, it checks s.OptionOverrides — if the user defined that key, it's skipped. Fully backward compatible: empty/nil options map produces identical behavior to the original code.

Test plan

• No overrides — all 5 defaults applied
• Empty overrides map — all defaults applied
• Single key override — that key skipped, others applied
• Multiple key overrides — those keys skipped
• Unrelated override (e.g. history-limit) — all defaults applied
• All 5 managed keys overridden — returns nil, no tmux call
• inject_status_line = false — returns nil (unchanged behavior)
• Full test suite passes with no regressions