feat: add per-remote LFS scoping via install --remote

[thevinchi]

Issue #, if available: #32

Description of changes:

git-lfs-s3 install sets lfs.standalonetransferagent globally, so git-lfs invokes git-lfs-s3 for every remote in the repo. Any push/pull to a non-S3 remote that uses LFS (GitHub, GitLab, etc.) fails because the agent rejects non-S3 URLs.

The root cause is that git-lfs parses s3:// as a bare SSH URL and attempts SSH endpoint discovery before consulting per-URL scoped agent config, so the existing workaround of writing lfs.<s3-url>.standalonetransferagent never takes effect (lfsapi/endpoint_finder.go:234-274, tq/manifest.go:270-284).

This PR adds git-lfs-s3 install --remote <name>, which sets remote.<name>.lfsurl to a synthetic HTTPS URL (https://lfs-alias.git-remote-s3.test/<bucket>/<prefix>). RemoteEndpoint() checks lfsurl before falling back to the remote URL, so the HTTPS-shaped value short-circuits SSH discovery. The scoped agent lookup then finds the per-URL standalonetransferagent and selects git-lfs-s3 for that remote only. The synthetic URL is never contacted (IsStandaloneTransfer() skips the LFS server entirely); the .test TLD is RFC 6761-reserved for non-resolvable use.

Changes:

• git_remote_s3/common.py — adds LFS_ALIAS_HOST constant and synthetic_lfs_url(bucket, prefix) helper.
• git_remote_s3/lfs.py
  • Refactors install() to accept an optional --remote <name> argument.
  • _install_unscoped() — preserves existing behavior; adds a stderr warning when more than one remote is configured.
  • _install_scoped() — writes remote.<name>.lfsurl, lfs.<url>.standalonetransferagent, and lfs.customtransfer.git-lfs-s3.path scoped to the named remote; errors if lfsurl is already set to a conflicting value; warns if unscoped lfs.standalonetransferagent is already set.
  • Switches both code paths from --add (non-idempotent) to --replace-all.
  • Adds _git_config_get, _git_config_set, _list_git_remotes, _resolve_s3_remote helpers.
• test/lfs_install_test.py (new) — 11 tests using real temp git repos:
  • Bare install, one remote → unscoped config written, no warning.
  • Bare install, multiple remotes → unscoped config written + warning.
  • --remote nonexistent remote → exit 1.
  • --remote non-S3 remote → exit 1.
  • --remote happy path (s3://) → three scoped keys, no global agent key.
  • --remote happy path (s3+zip://) → same.
  • Idempotency: re-running produces no duplicate values.
  • Conflicting pre-existing lfsurl → exit 1, original value preserved.
  • Pre-existing unscoped agent → warning emitted, scoped keys still written.
  • synthetic_lfs_url golden-value and RFC-reserved-TLD tests.
• README.md — updates LFS section to recommend --remote; documents the scoped install form and explains the synthetic URL; updates code examples.

Verification:

• pytest test/ — 59 passed (the pre-existing test_simultaneous_pushes_single_bundle_remains failure on main is unrelated to this change).
• black --check — clean on changed files.
• flake8 — clean on changed files.
• mypy — same 4 pre-existing errors as main; no new errors introduced.

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