Add init --sync and --prune for incremental monitor setup

`init` planned every file on every run and refused if any of them
existed, so attaching a new monitor could not be configured without
--force — which reallocated global workspace numbers by i3's current
output order and renumbered the monitors already in use.

- allocate_global_starts() keeps the block each known output already
  holds and gives new outputs the lowest free block; blocks of inactive
  outputs stay reserved.
- read_allocations()/parse_allocation() read those blocks back out of
  the generated files (inverse of render_workspaces_for_output).
- --sync writes only the missing files, --prune drops configs for
  outputs that are gone and frees their numbers.
- Files without the auto-generated header are never written or
  deleted, --force included.

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -103,10 +103,36 @@ Useful flags:
 | `-n N` / `--count N` | 8 | Workspaces per output |
 | `--config-dir PATH` | `~/.config/i3/pi3-smart-workspace` | Where to write |
 | `--dry-run` | off | Print to stdout, don't touch the filesystem |
-| `--force` | off | Overwrite existing files |
+| `--sync` | off | Only write configs for outputs that don't have one yet |
+| `--force` | off | Regenerate every file from scratch (renumbers workspaces) |
+| `--prune` | off | Delete generated configs for outputs that are no longer active |
 
-Re-run `init --force` after plugging or unplugging a monitor to
-regenerate the per-output folders.
+### Plugging in a new monitor
+
+Run:
+
+```sh
+pi3-smart-workspace init --sync
+```
+
+`--sync` writes a `workspaces.conf` only for the outputs that don't have
+one yet, and gives the newcomer the lowest block of global workspace
+numbers that isn't already taken. The monitors you already use keep the
+numbers they have, so your muscle memory survives — which a plain
+`--force` regeneration would not guarantee, since that allocates numbers
+by i3's current output order and can shuffle everything.
+
+Configs for monitors that are merely switched off are left in place and
+their numbers stay reserved. Add `--prune` to delete them and free those
+numbers for a new screen:
+
+```sh
+pi3-smart-workspace init --sync --prune
+```
+
+Files that don't carry the `# Auto-generated by pi3-smart-workspace init`
+header are never overwritten or deleted, not even with `--force` — so
+anything you hand-tune in that directory is safe.
 
 ### Example config (manual)