Add output size limits and sensitive-output warnings

[davidweb3-ctrl]

Add guardrails for large command output and potentially sensitive Git output so maintainers can use the server more safely in AI-assisted workflows.

Acceptance criteria:

• Define output-size limits for tools that can return large diffs, logs, or file lists.
• Warn when output may contain secrets, tokens, private remotes, or local filesystem details.
• Prefer truncation with clear continuation guidance over unbounded output.
• Include tests for truncation and warning behavior.