formatReadHashlineHeader collapsed every relative in-workspace path to its
basename, so reading a nested file (e.g. src/settings.json) emitted
[settings.json#tag]. When a same-basename file existed at the session cwd,
a verbatim follow-up edit resolved against the cwd file; Patcher.prepare only
runs snapshot-tag path recovery when the authored path is missing, so the
valid edit was deterministically rejected with "hash is not from this
session". Keep the workspace-relative path, which names the file uniquely and
stays directly resolvable against cwd. Out-of-workspace absolute paths remain
shortened; root-level files are unchanged.
Fixes#8482
roboomp
a02f88994b