Data dir defaults to ~/.gitbot-fleet — make it configurable (env / XDG) #580

gitbot-fleet defaults its state/data directory to ~/.gitbot-fleet, which on the owner's machine puts fleet data in $HOME instead of the workspace. Found by the 2026-09-30 local workspace audit (hypatia's fleet_path default also points under ~/Documents; see the companion issue in hypatia).

Acceptance criteria

• The data directory is configurable via an env var (e.g. GITBOT_FLEET_HOME), falling back to an XDG path ($XDG_DATA_HOME/gitbot-fleet), never a bare ~/.gitbot-fleet.
• Existing data is found at the old location on first run (migrate or warn), so nothing is lost.
• The variable is documented.

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