fix: restrict e to HEAD, add O to open historical files read-only (#2147)

[Metbcy]

This Pull Request fixes/closes #2147.

It changes the following:

• e (edit) on the Files-at-revision view is now only enabled when the revision being viewed is HEAD. Previously e always opened the current worktree copy regardless of which commit you were browsing, which was confusing and could lead to edits being misattributed to the wrong revision.
• A new O (shift+o) command is enabled only on non-HEAD revisions. It writes the blob at the selected revision into a fresh tempdir (prefixed with the full commit SHA), marks the file read-only, and opens it in the external editor.
• The tempdir is owned by the RevisionFilesComponent and is cleaned up when the next O replaces it or when the component is dropped, so there is no on-disk leak.
• On O, the view switches to the Status tab so it is visually clear that edits to the tempfile will not be persisted to the historical revision.

Testing

A new unit test (dumps_blob_to_readonly_tempfile_and_cleans_up_on_drop) covers the dump-to-readonly + drop-on-replace behavior. Full suite passes:

cargo nextest run --workspace
# 310 tests, 0 failed

I also smoke-tested manually on a small repo:

Scenario	Expected	Actual
At HEAD, e on a file	Opens worktree copy (writable)	✅ perms 644, real path
At HEAD, O on a file	Ignored	✅ no-op
At older commit, e	Ignored	✅ no-op
At older commit, O	Opens tempfile, read-only, historical content	✅ perms 444, /tmp/<sha><rand>/<file>, correct blob
Open a second file at the same revision	Previous tempdir dropped first	✅ old tempdir gone
Quit gitui	Tempdir removed	✅

Notes

• Previous attempt at this issue (Issue 2147 #2384) was closed by its author with the checklist unfinished; this PR is an independent implementation.
• is_head() returns false on lookup error, which keeps the safe behavior of preferring open over edit.

I followed the checklist:

• I added unittests
• I ran make check without errors (skipped deny/tombi locally CI will likely cover)
• I tested the overall application
• I added an appropriate item to the changelog