fix(use,delete,rename): support multiple directories per identity

[OfirHaim1]

The bug

Binding two directories to the same identity silently broke — the second binding clobbered the first.

Each includeIf block in ~/.gitconfig was keyed by the bare identity name (sentinel # >>> gitswitch:<name>), and use/rename strip the old block before writing the new one. So:

gitswitch use private ~/work        # writes block "gitswitch:private" → ~/work
gitswitch use private ~/site        # STRIPS it, writes "gitswitch:private" → ~/site

Now config.json records both bindings, but ~/.gitconfig holds one block. The un-blocked directory falls back to the global identity with no warning — gitswitch why/doctor would even report the binding as active because config.json looks right. Binding several repos to one identity (e.g. all your personal projects → personal) is completely normal, and it failed quietly.

The fix

Key each block per (identity, directory) instead — sentinel # >>> gitswitch:<name>:<dirhash> — so multiple directories can share an identity without clobbering.

• New syncBindingBlocks(cfg, name, path) rewrites all of an identity's blocks from config.json (the source of truth), one per bound directory, and removes the legacy bare-name block — migrating existing users forward on their next use/rename.
• use, delete, rename, and --unbind all route through per-binding keys / the sync helper.
• Idempotency fix in blocks: appendBlock now normalises trailing newlines (instead of only ever adding them) and stripBlock swallows the trailing blank separator. Without this, the repeated strip+re-append in syncBindingBlocks leaked a blank line into ~/.gitconfig on every run.

Before / after

# before — second bind wins, first dir reverts to global, no warning
$ git config --global --get-regexp includeif
includeif.gitdir:~/site/.path  .../private.gitconfig      # only one block

# after — one block per directory, both active
includeif.gitdir:~/work/.path  .../private.gitconfig
includeif.gitdir:~/site/.path  .../private.gitconfig

Test plan

• go build ./..., go vet ./... — clean
• go test ./internal/... — all pass, including new tests:
  • blocks: upsert/remove idempotency across many blocks (no blank-line leak)
  • cmd: multi-dir sync (one block per dir, legacy block removed, other identities untouched, idempotent); per-binding name distinct + stable
• Verified live: a real config with two directories bound to one identity now produces two includeIf blocks and both directories resolve to the correct identity; re-running use is byte-stable (no leaked blank lines).