docs(arch): Add ARCHITECTURE.md and CLAUDE.md, init repo

Bring the Blanton Tera Term TTL test suite under version control and
document how the pieces fit together.

- ARCHITECTURE.md: A/B/C script roles, host-to-DUT-to-FPGA data path,
  config/settings/profile data models, PCIe AER + EDAC topology table,
  10 key constraints, and 6 development phases derived from the
  Status_20260814 spreadsheet's On-Going items
- CLAUDE.md: stack, smoke-test commands, conventions, and the hardware
  footguns (per-unit BDF, VSPI vs VI2C timing, repeated START,
  STORE_USER_ALL, ARB_LOST false positives)
- secret/: gitignored credential store with README + .example template,
  so the DUT login stops living in 1_Blanton_Script_A.ttl
- .gitignore: secret/*, For_AI/, *.log, publish/, *.DSN

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014RetWKFZFG1ZHcitQAyhwM

secret/config_secret.ttl.example

@@ -0,0 +1,24 @@
+; =============================================================================
+; config_secret.ttl.example  --  DUT credentials (NOT in git)
+; =============================================================================
+; Usage:
+;   1. cp secret/config_secret.ttl.example secret/config_secret.ttl
+;   2. Replace every __CHANGE_ME__ below with the real value.
+;   3. Put config_secret.ttl next to config.ttl in the Tera Term working dir.
+;
+; config.ttl includes this file; 1_Blanton_Script_A.ttl must use the variables
+; below instead of a literal password:
+;
+;       sendln dut_user
+;       wait "Password:"
+;       sendln dut_password
+;
+; NEVER commit the filled-in config_secret.ttl.
+; =============================================================================
+
+; --- SONiC console login ---
+dut_user     = "__CHANGE_ME__"
+dut_password = "__CHANGE_ME__"
+
+; --- Optional: root password, if 'sudo -i' prompts on this unit ---
+dut_root_password = "__CHANGE_ME__"