Add "pkgwrap doctor" for diagnostics and bug reports

[trmxvibs]

Context

Every bug report needs the same information: OS, detected backend, whether
the cache is in play, Python version, pkgwrap version. Right now reporters
assemble that by hand, and the issue template asks for it piece by piece.

Proposal

$ pkgwrap doctor
pkgwrap 0.2.0 (Python 3.12.3, Linux 6.8.0)
Detected backend: apt (/usr/bin/apt)
Detection source: cache (~/.config/pkgwrap/backend.json, version 2)
Root: no    sudo: available
Colour: enabled    Unicode: enabled
Supported operations: install, remove, refresh, upgrade, search, list, info, clean

Rules: read-only, no sudo, never runs a package manager command, prints no
personal paths beyond the config directory.

Acceptance criteria

• pkgwrap doctor prints the report above
• Reports which operations the detected backend actually supports
• Says whether detection came from PKGWRAP_BACKEND, the cache, or a probe
• Tests for the detected-vs-cached and supported-operations logic
• Bug report template points at it

Pointers

• src/pkgwrap/detector.py — detection sources
• src/pkgwrap/backends/base.py — _unsupported() marks unsupported ops
• src/pkgwrap/ui.py — supports_color(), supports_unicode()