fix(mcp): reject local targets over HTTP transport

[rodboev]

Summary

HTTP MCP callers can currently pass local filesystem targets to scan_skill, and the server forwards those paths into the normal scan graph. This adds an MCP transport guard so routable HTTP use rejects local paths and file:// targets before the graph can read them, while CLI and stdio MCP scans keep their existing local-path behavior.

Closes #191

Root cause

scan_skill forwards the caller-provided target to run_scan without transport context. The input resolver then treats local files and directories as valid scan targets, which is correct for trusted local use but too broad for unauthenticated HTTP exposure.

Diff Notes

• Add an MCP-layer local-target guard for HTTP server calls.
• Preserve local filesystem scans for CLI and stdio/default run_scan use.
• Keep existing URL allowlist and private-IP protections in the input layer unchanged.
• Add focused MCP unit tests for local-target rejection, UNC-style path classification, remote-target allowance, and default local-target compatibility.

Scope

This does not add MCP authentication, change the generic input resolver, or alter report output. PR #193 covers the documentation warning slice; this PR covers the code guard.

Verification

• python -m pytest tests/unit/test_mcp_server.py -v -> 18 passed, 1 warning
• uv run ruff check src/ tests/ -> passed
• uv run ruff format --check src/skillspector/mcp_server.py tests/unit/test_mcp_server.py -> 2 files already formatted
• uv run ruff format --check src/ tests/ -> still reports pre-existing drift outside the target scope in src/skillspector/nodes/analyzers/static_runner.py, tests/nodes/analyzers/test_binary_and_pe3_filtering.py, tests/nodes/analyzers/test_mp2_regex_backtracking.py, and tests/nodes/test_llm_analyzer_base.py
• CI Lint & Test (Python 3.12), Lint & Test (Python 3.13), and DCO Check - pending maintainer approval if fork checks are gated

Notes

Follow-up to maintainer review on #36 (review).