Add env var for configurable base or root directory

[mcking-07]

Problem

Currently, all T3 Code data is hardcoded to ~/.t3 with no way to configure a different base location:

Path	Override?
~/.t3/userdata/ (state)	Yes T3CODE_STATE_DIR
~/.t3/dev/ (dev state)	Yes --state-dir
~/.t3/worktrees/	No (hardcoded)

Users who want to store T3 Code data in a different location (e.g., ~/.config/t3, encrypted volumes, network drives, or XDG-compliant paths) currently have no single config to control this.

Proposal

Add a T3CODE_BASE_DIR environment variable that serves as the root for all T3 Code paths:

T3CODE_BASE_DIR (default: ~/.t3)
├── userdata/     ← state (T3CODE_STATE_DIR takes precedence)
├── dev/          ← dev state
└── worktrees/    ← git worktrees

Behavior

• Default: ~/.t3 (no breaking change)
• Override: T3CODE_BASE_DIR=~/.config/t3 moves everything
• Backwards compat: T3CODE_STATE_DIR still takes precedence for state directory
• CLI flag: --base-dir for dev-runner and server

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Happy to implement if approved :)