Add mlr mcp MCP server + agent playbook, with a --no-shell gate (#2098 PR7) (#2133)

* Plan: flesh out PR7 (MCP server + Agent Skill) design

stdio transport (no HTTP port), mlr mcp terminal in the main binary,
SDK-vs-handroll decision, tool list, in-process vs subprocess split,
run-tool safety (--no-shell prerequisite), single-sourced skill, tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Add mlr mcp: MCP server + agent playbook; --no-shell gate (#2098 PR7)

New terminal `mlr mcp` runs a Model Context Protocol server over stdio
(spawned by MCP clients; no network port), exposing five tools --
list_capabilities, which, validate_dsl, describe_data, run -- plus an
agent playbook as MCP prompt/resource. Catalog tools are served
in-process from the help registries; the rest subprocess this same
binary with MLR_ERRORS_JSON=1, a timeout, and an output cap.

Prerequisite: a new --no-shell flag / MLR_NO_SHELL env var (one-way
gate) disables the DSL system/exec functions, piped redirects, and
--prepipe/--prepipex; the MCP server sets it on the commands it runs
unless started with --allow-shell.

Adds the github.com/modelcontextprotocol/go-sdk dependency.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Force LF checkout for the embedded SKILL.md (Windows CI fix)

go:embed embeds checkout bytes, so a CRLF checkout on Windows made the
embedded playbook differ per platform and failed
TestPlaybookHasFrontmatter. Pin the file to eol=lf in .gitattributes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Move no-shell test DSL into per-case mlr files (Windows CI fix)

Inline single-quoted DSL in cmd files is mangled by the Windows shell
(single quotes are not quote characters there); the harness's
put -f ${CASEDIR}/mlr pattern avoids shell quoting entirely.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs/src/manpage.md

@@ -652,6 +652,16 @@ This is simply a copy of what you should see on running `man mlr` at a command p
                                 since direct-to-screen output for large files has its
                                 own overhead.
        --no-hash-records        See --hash-records.
+       --no-shell               Disable Miller's ability to run external commands:
+                                the DSL `system` and `exec` functions, piped
+                                redirects such as `tee | "command"`, and
+                                `--prepipe`/`--prepipex` all fail cleanly instead of
+                                executing. Equivalent to setting the `MLR_NO_SHELL`
+                                environment variable to a truthy value. Intended for
+                                running agent-constructed command lines (e.g. via
+                                `mlr mcp`) without also granting arbitrary command
+                                execution. Once disabled, shell-outs cannot be
+                                re-enabled for the rest of the process.
        --norc                   Do not load a .mlrrc file.
        --nr-progress-mod {m}    With m a positive integer: print filename and record
                                 count to os.Stderr every m input records.