chore(board): rename $BOARD_EDITOR to $DOCKER_AGENT_BOARD_EDITOR

[dgageot]

The board feature lets users configure a preferred editor via an environment variable, but the original name BOARD_EDITOR is too generic and doesn't follow the DOCKER_AGENT_ prefix convention used by the project's other env vars.

This renames the variable to DOCKER_AGENT_BOARD_EDITOR across the codebase and documentation. To avoid silently breaking existing setups, the old BOARD_EDITOR name is kept as a fallback for one release — both in the Go env lookup and in the tmux shell expansion (${DOCKER_AGENT_BOARD_EDITOR:-${BOARD_EDITOR:-code}}), matching the pattern already used for other renamed env vars in this project.

Users relying on BOARD_EDITOR will continue to see their editor launched without any change; they can migrate to DOCKER_AGENT_BOARD_EDITOR at their own pace before the fallback is removed.

[docker-agent]

Assessment: 🟢 APPROVE

This is a clean, well-structured rename of BOARD_EDITOR → DOCKER_AGENT_BOARD_EDITOR. The backward-compatibility fallback chain is correctly implemented in both the Go env lookup and the tmux shell expansion, and documentation is updated consistently. No bugs or regressions introduced.