Support user tmux configuration (.tmux.conf)

[h0x91b]

Problem

dev-3.0 spawns tmux sessions for each task, but currently uses its own default tmux configuration. Users who have customized their ~/.tmux.conf (keybindings, status bar, mouse settings, plugins like tpm, etc.) lose all of that inside dev-3.0 sessions. This creates a jarring experience — the terminal feels "alien" compared to their normal workflow.

Proposed solution

Respect the user's existing tmux configuration when creating sessions. Options to consider:

1. Source user's config by default — when spawning a tmux server/session, pass -f ~/.tmux.conf (or detect the config location) so the user's settings apply automatically.
2. Per-project override — allow projects to specify a custom tmux config path in project settings, overriding the user's default when needed (e.g., for a project that requires specific tmux layout).
3. Merge approach — source the user's config first, then apply any dev-3.0-specific overrides on top (e.g., if dev-3.0 needs certain options for its integration to work).

Things to consider

• Some user configs may conflict with dev-3.0's tmux integration (e.g., custom prefix key, status bar overrides, or plugins that rebind keys dev-3.0 relies on).
• Need to handle the case where ~/.tmux.conf doesn't exist (no-op, use defaults).
• tmux plugin managers (tpm) may need special handling — run-shell commands could fail in the dev-3.0 context if paths differ.
• Should there be a "reset to dev-3.0 defaults" escape hatch if the user's config breaks something?