feat: accept full browser URLs (tree URLs, blob rejection, --ref / --subpath)

[raiderrobert]

Problem

Shortcodes remove parsing ambiguity (which is their value), but they force users to mentally translate a URL from their browser into gh:user/repo/subpath form. The natural workflow is: navigate to the template in the browser, copy the URL bar, paste it. That should just work.

Proposed feature

Accept full https:// URLs that follow vendor-specific tree-URL patterns and parse them into (url, ref, subpath):

• GitHub: https://github.com/OWNER/REPO/tree/REF/PATH
• GitLab: https://gitlab.com/OWNER/REPO/-/tree/REF/PATH
• Codeberg: https://codeberg.org/OWNER/REPO/src/branch/REF/PATH

Additional behavior:

• Plain repo URLs (no /tree/...) clone at default branch, repo root — works today, keep working
• blob URLs (file views) → rejected with a clear error: "URL points at a file, not a directory — copy from a folder view instead"
• Branch names can contain slashes (e.g. feature/foo). Disambiguate by running git ls-remote once before clone and using longest-prefix match against the known refs. Fall back to single-segment-ref interpretation if offline.
• Add --ref <REF> and --subpath <PATH> CLI flags on diecut new as escape hatches for self-hosted / enterprise / unknown vendors
• If a tree URL and --ref/--subpath disagree → error: "ambiguous source — remove one"
• Non-big-three vendors work as plain URLs, using the flags for ref/subpath

Non-goals

• Parsing Bitbucket or other vendor URL patterns (plain URL + flags instead)
• Replacing shortcodes (shortcodes stay — see Drop shorthand abbreviations, accept full URLs only #128 discussion)

Acceptance criteria

• Tree-URL parser for GitHub, GitLab, Codeberg
• ls-remote-based ref disambiguation with offline fallback
• --ref and --subpath flags on diecut new
• Blob URL rejection with helpful error
• Conflict detection when tree-URL and flags disagree
• Integration tests: plain URL, tree URL, multi-slash branch, blob URL, conflict error, escape-hatch flags
• Docs updated