Replace create --hostname/--username with BORG_HOSTNAME/BORG_USERNAME env vars, fixes #9651

[ThomasWaldmann]

Forward-port of the 1.4-maint env-var approach (#9810) to master, and removal of the borg create --hostname/--username CLI options (added in 2.0.0b21, #9402).

Per maintainer decision, hostname/username overriding is now env-vars only:

• BORG_HOSTNAME — overrides the hostname
• BORG_USERNAME — overrides the username

When set, the override is used both for the value stored in newly created
archives and for the {hostname} / {user} placeholders (so it also
applies to archive names, prune --glob-archives '{hostname}-*', check, etc.).

Use case (#9651): run borg on host A but impersonate host B — e.g. a reliably
online machine performs prune/compact/check on behalf of an opportunistically
online laptop against a shared, deduplicated repository, with consistent
per-hostname archive naming.

Fixes #9651.

Changes

• Removed the --hostname/--username options and the corresponding
  Archive.__init__ parameters.
• Archive.save() now stores os.environ.get("BORG_HOSTNAME") or platform.hostname
  and os.environ.get("BORG_USERNAME") or getuser().
• _replace_placeholders() honors the same overrides for {hostname} / {user}.
• fqdn, hostid (lock stale-removal, see BORG_HOST_ID) and the normal
  auto-detection are intentionally left untouched.
• Docs: documented both env vars; regenerated create usage; changelog entry
  under 2.0.0b22.
• Test renamed to test_hostname_and_username_override, now driving the feature
  via env vars and also asserting {hostname}-{user} placeholder substitution in
  the archive name.

Note

This is a breaking change relative to the 2.0.0b21 beta for anyone who used
--hostname/--username, but beta releases are explicitly test-only, so this is
acceptable.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.77%. Comparing base (4a28995) to head (5e27122).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9811   +/-   ##
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  Coverage   84.77%   84.77%           
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  Files          92       92           
  Lines       15289    15285    -4     
  Branches     2297     2297           
=======================================
- Hits        12961    12958    -3     
+ Misses       1630     1629    -1     
  Partials      698      698           

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