Closes #885.

Problem

cli.py printed exception text with err_console.print(f"[red]Error:[/red] {e}"), so any square brackets in the message were parsed as Rich markup:

• skillspector scan "skills[old]/demo" reported Cannot determine input type for: skills/demo (the [old] was swallowed).
• skillspector scan "nope[/x]" crashed with MarkupError and exit code 1 instead of the usual exit code 2.
• skillspector mcp without the extra printed pip install 'skillspector', dropping [mcp] from the install hint.

#725 and #734 fixed the same class of bug for terminal reports and batch output; the CLI error lines were not covered.

Change

Add _error_line() in cli.py. It builds the line from a rich.text.Text object and passes each line of the message through the shared visible_terminal_text helper (already imported as _visible_terminal_text), following the approach reviewed in #725 and #734:

• no markup parsing, so brackets and tags show literally
• no emoji replacement and no doubled trailing backslashes (a Text object is not markup-escaped)
• control characters such as ESC are shown as visible escapes
• newlines are kept, so the "Supported formats: ..." second line stays on its own line

Twelve call sites that interpolated an exception message now use err_console.print(_error_line(...)) (line numbers on main): 397, 702, 753, 766, 773, 828, 834, 924, 930, 3419 (the skillspector mcp handler), 3523 and 3529.

I left alone the Error: lines that are fixed string literals (nothing to inject), and the recursive child-scan message, which prints a constant with a custom indent.

Tests

13 new tests in tests/unit/test_cli.py:

• _error_line rendered literally: plain text, skills[old]/demo, nope[/x], [bold]x[/bold], the skillspector[mcp] hint, an emoji shortcode, a trailing backslash, a multi-line message, and an ESC sequence (9 cases)
• _error_line survives Console.print (1)
• skillspector scan on skills[old]/demo and nope[/x] through CliRunner: exit code 2, bracketed text intact, no MarkupError (2)
• skillspector mcp with the extra missing keeps skillspector[mcp] in the hint (1)

On the unmodified code, the 10 _error_line tests fail because the helper does not exist; the three end-to-end tests fail for the bug itself:

• the two scan tests fail with the lost [old] and the MarkupError / exit 1
• the MCP test fails with pip install 'skillspector' printed instead of skillspector[mcp]

make lint and make format-check pass.

tests/unit/test_cli.py on its own, with all 13 new tests: 427 passed.

Full unit suite (make test-unit, run before the MCP-hint test was added): 14,739 passed, 6 failed, 35 skipped, 4 xfailed. All 6 failures are test_native_claude_55_accepts_default_temperature_in_request_and_provenance in tests/unit/test_anthropic_provider.py (KeyError: 'temperature'). They fail identically with my change stashed, so they are unrelated.

Behaviour notes

• A tab in an error message now shows as \x09 and a carriage return as \x0d (same helper as the terminal report).
• The error text no longer goes through Rich's automatic number and path highlighting, because it is a Text object rather than a plain string. This is cosmetic.

I did not touch CHANGELOG.md; happy to add a line if you want one.