feat(cmd): add doctor command for diagnosing configuration issues

[CalvinAllen]

Summary

Adds dtvem doctor, a diagnostic command that runs nine checks against a dtvem installation and reports findings as [fixable] (doctor can remediate) or [manual] (step-by-step instructions). Default mode is read-only; --fix enables interactive remediation, and --yes skips per-finding prompts. Exits non-zero when any error-severity finding is present so CI scripts can react.

Checks

Check	Severity	Remediation
stale-shims-path	error	fixable (Windows User PATH registry / Unix shell-config marker blocks)
shims-in-path	error	manual (dtvem init)
shim-binary-present	error	manual (reinstall)
empty-shims-directory	error	fixable (Rehash)
shim-map-cache	warning	fixable (Rehash, with post-fix drift check that flags orphan shim files Rehash can't resolve)
system-runtime-precedence	warning	manual (uninstall / reorder PATH / accept)
xdg-split-state	warning	manual (consolidate install)
configured-runtimes-installed	error	manual (dtvem install <r> <v>)
runtime-executable-present	error	manual (uninstall+install for first problem)

Framework

src/internal/doctor/ contains the framework — Check interface, Finding/Detail/Severity types, a Registry with init()-time registration (same pattern as runtime/migration providers), and a Run aggregator with HasErrors() and Fixable() helpers. Adding a new check is one Go file plus a Register(...) call in init().

Path helpers

Two new helpers in internal/path back the stale-shims fix:

• RemoveStaleShimsFromUserPath (Windows-only): rewrites the User PATH registry value to drop stale dtvem-shims entries while preserving every other entry's order. No admin required. Stale entries in System PATH still need an elevated terminal and aren't auto-touched.
• RemoveStaleShimsFromShellConfig (cross-platform, called only from Unix branch): only deletes pairs of lines where the first is exactly the # Added by dtvem marker, so user-edited PATH exports are never touched even if they happen to reference an old dtvem path.

Both share a new pure helper PartitionStaleShimsEntries, which complements the existing FindStaleShimsEntries.

Test plan

• ./rnr check passes (format, lint, all tests)
• All 9 checks have unit tests covering pass/fail/edge cases
• dtvem doctor runs all checks read-only and prints a report
• Findings render with [fixable] / [manual] tags
• dtvem doctor --fix --yes applies fixable findings non-interactively
• dtvem doctor --fix --no-fix errors with exit code 2
• Smoke-tested locally on Windows: detected a real shim-map drift case (python3.exe shim with no installed-version executable to back it); the post-fix drift check correctly surfaced the orphan filename rather than falsely claiming Fixed
• Smoke-test on macOS / Linux

Resolves #248