feat: enhance known_hosts management with deletion cleanup and SSH cache handling

DEVELOPER.md

@@ -77,6 +77,7 @@ Packet integration tests must remain inside disposable isolated namespaces.
 │   ├── integrity.py          # SHA-256 and integrity manifest logic
 │   ├── install.py            # Apt, Firecracker, kernel, rootfs installation
 │   ├── images.py             # Template lookup, private disks, and guest credentials
+│   ├── known_hosts.py        # Deletion-only cleanup of local SSH host-key caches
 │   ├── network.py            # IP/MAC/TAP/bridge/NAT/forward rules
 │   ├── firewall.py           # Owned nft bridge tables and offline conntrack cleanup
 │   ├── lifecycle.py          # Lifecycle, bulk operations, recovery, port intent
@@ -185,6 +186,7 @@ process:
 | CommandRunner | `CommandRunner` | Logged/testable subprocess execution |
 | StateStore | `StateStore` | Inventory, locks, JSON persistence |
 | ImageStore | `ImageStore` | Template verification, VM disks, and offline credentials |
+| KnownHostsManager | `KnownHostsManager` | Scoped local SSH cache cleanup before deleted IPs are released |
 | NetworkManager | `NetworkManager` | Guest allocation and Linux networking |
 | FirewallManager | Constructed by `NetworkManager` | Owned native bridge policy and scoped offline conntrack cleanup |
 | FirecrackerProcessManager | `FirecrackerProcessManager` | VMM process control |
@@ -415,7 +417,7 @@ stop:
 
 delete (destroy alias):
   persist delete intent -> same runtime cleanup -> remove owned runtime directory
-  -> remove state record -> release name/IP
+  -> clean local known_hosts entries -> remove state record -> release name/IP
 ```
 
 Stop signals the **VMM**, not a guest shutdown agent or ACPI-coordinated
@@ -428,6 +430,28 @@ Runtime cleanup retains reservations when process exit, TAP ownership, or
 connection cleanup is uncertain. A pending spawn cannot be declared gone
 merely because its PID was never persisted or its socket has not appeared.
 
+`KnownHostsManager` is an explicit lifecycle dependency, also used by API and
+bulk deletion. It runs after successful process/network/disk teardown and before
+the registry entry is removed, under the same operation lock. Cleanup errors
+retain `operation=delete` and the IP reservation; repeating delete retries it.
+Stop, restart, and resize retain the existing guest's trusted host-key entries.
+
+Default caches are `known_hosts` and `known_hosts2` under the effective user's
+and sudo invoker's account homes (`SUDO_UID`, with a `SUDO_USER` fallback).
+`UVM_KNOWN_HOSTS_FILES` overrides this discovery for service deployments using a
+colon-separated list of absolute paths. Only local files are managed. Matches
+are explicit guest IPs or UVM's `uvm-<VM-ID>` host-key alias, including OpenSSH's
+HMAC-SHA1 hashed form. Plain bracketed ports are recognized directly; hashed
+bracketed candidates use port 22 and the VM's recorded allowlist. Wildcard rules
+and other aliases sharing a line are retained.
+
+File updates use pinned directory descriptors, regular/single-link checks,
+an advisory lock, and a sibling temporary file. Owner, group, mode, and extended
+attributes are preserved. A changed source snapshot prevents publication over
+concurrent edits. A matching-free file is not rewritten and missing files are
+not created. Tests inject the dependency to avoid accessing developers' caches,
+and verify hashed-entry interoperability using real OpenSSH on temporary files.
+
 ### Policy and Bulk Operations
 
 `set_ports()` takes a frozen registry ID, not a reusable name/IP; the CLI resolves