feat(shortcuts): regex rewrite rules for host shortcuts

shortcuts.rewrites applies regex replacements after the domain suffix
is stripped, e.g. ^([a-z]+)[0-9]+- -> $1 turns fsn1-bx1.hoster.de into
fsnbx1. The shortcut works for connecting (cached hosts), shell
aliases, and Tab completion, where it is offered when the full name
does not match the typed text. Rules are editable in netssh configure,
and netssh alias --test HOSTNAME shows a name's shortcut and the cached
hosts it matches.

The shortcut lookup only ran when domains or strip_hyphens were set;
it now also runs for rewrite-only configurations.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -29,7 +29,7 @@ netssh -p 2222 admin@app-server-03 uptime
 - **Shell completion** — install without sudo via `netssh completion install`
 - **Default SSH user** — set a fallback username once in config instead of typing it every time
 - **Default SSH port** — set a fallback port in config; injected as `-p` when not specified on the command line
-- **Host shortcuts** — type `web01` instead of `web01.example.com`; configurable domain strip and hyphen folding
+- **Host shortcuts** — type `web01` instead of `web01.example.com`; configurable domain strip, hyphen folding, and regex rewrite rules (`fsn1-bx1.hoster.de` → `fsnbx1`)
 - **Shell aliases** — `netssh alias` generates shell aliases for all cached hosts
 - **Automatic 24h cache refresh** — the cache is refreshed in the background every 24 hours
 - **Jump hosts and SSH options** — `proxy_jump`, host key checking, connection target, and free `-o` options per netssh, per NetBox instance, or per tag/name rule — without touching `~/.ssh/config`
@@ -115,7 +115,7 @@ A menu then gives access to every other setting; open only the sections you want
 | SSH options | `proxy_jump`, `host_key_checking`, `target`, `address_family`, `extra_args` (one per line), `options` (`Key=Value` per line) |
 | SSH rules | add, edit, remove `ssh.rules` — match by tag, instance, kind, name pattern, each with its own SSH options |
 | Resolver & cache | strategies, management subnets, interface name, cache TTL, cache file |
-| Shortcuts & command alias | domain suffixes, hyphen stripping, `shell.alias` |
+| Shortcuts & command alias | domain suffixes, hyphen stripping, rewrite rules (`REGEX => REPLACEMENT` per line), `shell.alias` |
 | Completion | `completion.max_results` |
 
 The result is validated and saved to `~/.config/netssh.yaml`; Ctrl+C cancels without writing.
@@ -366,13 +366,26 @@ shortcuts:
     - .example.com
     - .example.de
   strip_hyphens: false  # optional
+  rewrites:             # optional, applied in order
+    - pattern: '^([a-z]+)[0-9]+-'   # fsn1-bx1 → fsnbx1
+      replace: '$1'
 ```
 
 Resolution order:
 
 1. **Exact match** — the input is looked up as-is in the cache.
 2. **Domain expansion** — if the input contains no dot, each configured domain is appended and tried (`web01` → `web01.example.com`). If no cache hit, NetBox is queried with the expanded name.
-3. **Hyphen folding** (`strip_hyphens: true`) — hyphens are inserted back before looking up in the cache (`fsn1web01` → `fsn1-web01.example.com`). This only works when the host is already cached; run `netssh cache refresh` first.
+3. **Shortcut match** — the input and every cached hostname are shortened the same way and compared: lowercase, strip the domain suffix, apply each rewrite rule in order, then remove hyphens if `strip_hyphens` is set (`fsn1web01` → `fsn1-web01.example.com`, `fsnbx1` → `fsn1-bx1.hoster.de`). This only works when the host is already cached; run `netssh cache refresh` first.
+
+**Rewrite rules** use Go regular expressions ([syntax](https://pkg.go.dev/regexp/syntax)); `replace` may refer to groups as `$1` or `${1}`. Every match in the name is replaced. Because the typed shortcut is rewritten too, write rules that leave an already shortened name unchanged: `^([a-z]+)[0-9]+-` matches `fsn1-bx1` but not `fsnbx1`. Shortcuts also become shell aliases and are offered by Tab completion when the full name does not match what you typed (`fsnb<Tab>` → `fsnbx1`).
+
+Check what a hostname is shortened to, and whether the shortcut is unique in the cache:
+
+```sh
+$ netssh alias --test fsn1-bx1.hoster.de
+fsn1-bx1.hoster.de → fsnbx1
+matches fsn1-bx1.hoster.de
+```
 
 Domains may be configured with or without a leading dot. If several cached hosts normalize to the same shortcut, the connection is rejected; use the full hostname. Live host lookups check every result page and reject duplicate exact names.
 
@@ -380,6 +393,7 @@ Domains may be configured with or without a leading dot. If several cached hosts
 |-------|--------|-------------|
 | `web01` | `domains: [.example.com]` | `web01.example.com` |
 | `fsn1web01` | `domains: [.example.com]`, `strip_hyphens: true` | `fsn1-web01.example.com` (cache only) |
+| `fsnbx1` | `domains: [.hoster.de]`, rewrite `^([a-z]+)[0-9]+-` → `$1` | `fsn1-bx1.hoster.de` (cache only) |
 
 ### Shell aliases