feat(cli): MCP server mode via structcli WithMCP

[leodido]

Stacked on #40. Review #38 → #39 → #40 first; this PR's diff is minimal once those land. The base will retarget to main automatically when #40 merges.

Why

structcli v0.17.0 ships an --mcp capability (pure stdlib JSON-RPC over stdio, no extra heavy SDK dependency, ~13 KB binary impact) that turns any Bind-driven cobra tree into a Model Context Protocol server. With #38–#40 in place — typed flags, structured errors, semantic exit codes, JSON Schema discovery — kfeatures is exactly the shape MCP wants: every runnable leaf becomes a tool whose schema mirrors the cobra flag set, and any failure produces a JSON envelope an agent can route on.

This PR wires it up.

What changes

1. structcli.WithMCP(...) added to the Setup call. Tools exposed: probe, check, config. Excluded: version (build metadata is in MCP serverInfo) and completion-bash/zsh/fish/powershell (no agent value).
2. mcpServerVersion() mirrors the human-facing version output so MCP clients can correlate tool output with build metadata. Falls back to "dev" when built without ldflags, matching the existing version subcommand behaviour.
3. Stream routing. Every command handler now writes through cmd.OutOrStdout() / cmd.ErrOrStderr() instead of bare os.Stdout / os.Stderr / fmt.Print*. Required so structcli's per-call buffer capture works; non-MCP behaviour is bit-for-bit identical because cobra's OutOrStdout() resolves to os.Stdout when no SetOut was called. printJSON gained an io.Writer parameter.
4. Session survival. Added inMCPMode(c) helper (detects Out != os.Stdout) and replaced os.Exit(1) with return err for the FeatureError path in check and the "kernel config not available" path in config. os.Exit would terminate the MCP server mid-session and kill subsequent tools/call requests on the same stdio connection.
5. Under MCP, the check FeatureError carve-out is collapsed: skip the --json {ok,feature,reason} payload and the FAIL: … print, because the MCP layer discards command stdout/stderr on execErr and emits the structcli error envelope as isError=true content. The agent receives the verbatim message in the envelope; emitting the carve-out shape would be wasted work and confusing (two competing JSON shapes for the same outcome).

Behaviour matrix

Surface	CLI mode	MCP tools/call
probe	unchanged	content.text = probe output (text or JSON per arguments.json)
check --require bpf-syscall	exit 0, "OK: …"	isError=false, content.text = "OK: …"
check --require bpf-fs (FAIL)	exit 1, FAIL: bpf-fs — … on stderr	isError=true, content.text = structcli envelope (exit_code: 1)
check --require bpf-fs --json (FAIL)	exit 1, {ok:false,feature,reason} stdout	isError=true, structcli envelope (carve-out collapsed)
check --require nonexistent	exit 11, invalid_flag_value envelope	isError=true, same envelope as content
config when /proc/config.gz missing	exit 1, "kernel config not available"	isError=true, content.text = same message
version	unchanged	not exposed (excluded; serverInfo.version is the agent-facing form)
completion <shell>	unchanged	not exposed (excluded)

Tests

New test/cli_mcp.bats (8 tests):

• --mcp persistent flag exists on root.
• initialize returns protocolVersion 2024-11-05 and the expected serverInfo.
• tools/list exposes exactly [check, config, probe] (excludes verified).
• inputSchema for check includes require and json properties.
• Invocation errors flow through the structcli envelope (isError=true, exit_code=11 inside).
• tools/call for an excluded tool returns a JSON-RPC error.
• 4-request session (init + ok + FeatureError + probe) — server stays alive, all responses well-formed.
• Stdout leakage guard: every line of stdout in a multi-call session must parse as JSON-RPC.

go test ./... clean. cli_common.bats + cli_linux.bats + cli_mcp.bats = 34/34 green.

Out of scope

• README / CONTRIBUTING surfacing of MCP usage and the structured-errors story (PR 4).
• Optional: an examples/mcp/ claude_desktop_config.json snippet — defer until after the user-facing docs PR lands so we can link from one place.