feat: add alias creation to pos system bank interactive menu

New 'alias' subcommand (+ POS_SUBCMDS row, menu option 6 'Manage
aliases'): create/update/list/remove bash aliases that run
'pos system bank run <name>', written into ~/.bashrc inside a managed
marker block (BASH_RC_FILE seam for tests, atomic tmp+mv rewrite).

- alias_valid_name enforces ^[a-zA-Z][a-zA-Z0-9_-]*$
- same-name create is an idempotent retarget; last alias removal cleans
  the whole block; outer ~/.bashrc aliases preserved byte-identically
- outer-alias collision refused (file untouched), PATH-shadow non-blocking
  warn, 'bank remove' drops aliases pointing at the removed command
- GNU sed '1,0p' edge case (block at line 1) fixed with guarded ranges;
  regex-quote literalization avoided via glob case payload match
- t-bank.sh +49 checks (71 -> 120); docs: POS.md, howto/system.md,
  tests/README.md, AGENT_TODO.md; make gen byte-idempotent
  (also refreshes the pos-communication-telegram-listener filetable
  line count so the previous commit's tree is gen-consistent)

completions/pos.bash

@@ -50,7 +50,7 @@ _pos_subcmds[share-nfs-server]="status share unshare list reload enable disable
 _pos_subcmds[share-smb-client]="mount unmount list persist unpersist menu"
 _pos_subcmds[share-smb-server]="status share unshare list adduser deluser reload enable disable menu"
 _pos_subcmds[system-backup]="menu"
-_pos_subcmds[system-bank]="list add show run edit remove"
+_pos_subcmds[system-bank]="list add show run edit remove alias"
 _pos_subcmds[system-schedule]="run list config enable disable status migrate menu"
 _pos_subcmds[ai]="ask chat sessions capture models providers llamacpp alias gemini hf openrouter server"
 # GEN:END possubcmds