fixpath: generalize prefix stripping with path extraction regex

[djdarcy]

fixpath: generalize prefix stripping instead of hardcoded PS/quote checks

Problem

fixpath currently has specific checks for PowerShell prefix (PS ) and surrounding quotes. But the real pattern is more general: extract the actual path from whatever wraps it.

Current code:

# Strip PowerShell prompt prefix
if path.upper().startswith("PS "):
    rest = path[3:].lstrip()
    if rest and (rest[0] in "/\\~" or (len(rest) > 1 and rest[1] == ":")):
        path = rest

# Strip surrounding quotes and backticks
if len(path) >= 2:
    if (path[0] == '"' and path[-1] == '"') or ...

These are special cases of a generic problem: "there's a real path embedded in this string, extract it."

Proposed solution

Replace the hardcoded prefix/suffix checks with a generic path extractor that finds the first valid path pattern in the input string:

# Match a drive-letter path (C:\...) anywhere in the string
m = re.search(r'([a-zA-Z]):\\', path)
if m and m.start() > 0:
    # Everything before the drive letter is prefix noise
    path = path[m.start():]

# Also match Unix-style paths (/home/..., ~/..., /c/...)
# and UNC (\\server\...)

This would handle:

• PS C:\code\file.md -- PowerShell prompt
• >>> C:\code\file.md -- Python REPL
• In [1]: C:\code\file.md -- IPython/Jupyter
• user@host: C:\code\file.md -- SSH prompt copy-paste
• "C:\code\file.md" -- quotes (the drive letter is inside)
• `C:\code\file.md` -- backticks

The surrounding-character stripping (quotes, backticks) should still happen first as a cheap check, but the prefix stripping should be the generic regex approach.

Design considerations

• The regex [a-zA-Z]:\\ is safe for Windows paths -- a single letter followed by :\ is unambiguous
• For Unix paths, finding /home/ or ~/ or /mnt/ in a string is less reliable (could be part of a URL or sentence)
• Keep quote/backtick stripping as a fast pre-pass (handles the common case without regex)
• The generic extractor should be a fallback, not the first thing that runs
• Consider: what if the "prefix" IS part of the path? e.g., PS C:\ could theoretically be a folder named PS C:\ -- but that's not a realistic scenario

Acceptance criteria

• Generic prefix extraction replaces hardcoded PS check
• Handles Python REPL (>>>), IPython (In [N]:), SSH prompts
• Quote/backtick stripping remains as fast pre-pass
• No false positives on paths that legitimately start with text before a drive letter
• Unix path extraction handled where unambiguous (~/, /home/, /mnt/)
• Existing tests still pass