security: remove node credentials from tracked files (open-source Phase 1)

Scrubs the fleet SSH/UI password from every tracked file (22 occurrences)
and removes inline credentials from the code paths that used them.

Docs and trackers keep the surrounding context — these are published under
docs/history/ per the open-source plan — with the literals replaced by
<FLEET_PW> / <FLEET_PW_ALT> so the "two variants exist" detail survives
without the values.

Three of the eight files were in .planning/ and were NOT in the plan's
enumerated list; the reworked audit-secrets.sh found them.

Code changes:
- neode-ui/test-openwrt.mjs: node URL and password come from ARCHY_NODE_URL /
  ARCHY_NODE_PW; the SSH target derives from the URL instead of a hardcoded
  tailnet IP; exits 2 when unset.
- scripts/run-post-install-tests.sh: drops the built-in "testpass123!"
  default and adds --password-stdin; refuses to run unauthenticated instead
  of silently trying a known password. --phase1-only still needs no password.
- .gitea/workflows/post-install-tests.yml: sshpass with an inline literal
  replaced by key auth (NODE_SSH_KEY secret); password comes from the
  NODE_UI_PASSWORD secret and is piped over stdin rather than argv, so it
  stays out of the node's process list and the job log. Default target IP
  removed.

scripts/audit-secrets.sh now reports 5/5 pass, 0 fail.

Note: rotation of the exposed credentials is deliberately deferred to the
pre-publish gate and is NOT done by this commit — these values are still
live. See Phase 0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/multinode/meshtastic.sh

@@ -23,7 +23,7 @@
 #
 # Nodes override via env (each must have a Meshtastic radio on the SAME LoRa
 # channel/region so they can actually hear each other):
-#   MA_URL MA_PW   node A  (sender)     default .116 http  / ThisIsWeb54321@
+#   MA_URL MA_PW   node A  (sender)     default .116 http  / <FLEET_PW>
 #   MB_URL MB_PW   node B  (receiver)   default .228 https / password123
 #   MC_URL MC_PW   node C  (eavesdrop)  OPTIONAL — enables privacy test (4)
 #