docs: sandbox template contents and config version hint

[aheritier]

Documents user-visible changes from PRs merged in the last 36 hours.

PR #3545 — install docker-mcp plugin in sbx template:
Added a "What the default template includes" subsection to docs/configuration/sandbox/index.md noting that docker-mcp is pre-installed in the default sandbox template, so agents can invoke docker mcp commands inside the sandbox without additional setup.

PR #3550 — config key requires newer schema version hint:

• Updated the config version in docs/configuration/overview/index.md from 10 to 12 (current schema, confirmed by agent-schema.json).
• Added a short paragraph explaining that Docker Agent appends a helpful hint to strict-parse errors pointing users to the minimum required schema version.

PRs already self-documented (included docs changes in their own commits):

• feat(setup): add custom OpenAI-compatible providers usable across commands #3558 (custom provider setup): docs/providers/custom/index.md global providers section
• feat(board): let the user configure their own columns #3556 (board columns): docs/features/board/index.md
• feat(skills): discover skills in .github/skills directories #3555 (skills .github/skills): docs/features/skills/index.md, docs/configuration/sandbox/index.md
• feat(tui): steer messages into the ongoing stream, add a /settings send-mode switch #3553 (steer/settings): docs/features/tui/index.md
• feat(tui): on-hover copy button on all messages and on-hover edit button on user messages #3559 (hover copy/edit): docs/features/tui/index.md
• fix(config): accept a single mapping for hook event lists #3546 (single-mapping hooks): docs/configuration/hooks/index.md