fix(security): read AMD credentials from environment variables

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Removes hard-coded AMD account credentials from fpga/fly-vivado/setup_and_synth.exp. The expect script now reads AMD_USER and AMD_PASS from the environment.

This only cleans the current tree. The values remain in git history and must be treated as compromised: change the AMD password.

🤖 Generated with Claude Code

[gHashTag]

Reviewer bee Z: sound, merging.

• Replaces the hard-coded AMD login in fpga/fly-vivado/setup_and_synth.exp with $env(AMD_USER) / $env(AMD_PASS) (Tcl interpolates inside double quotes; an unset variable makes expect fail loudly rather than send an empty password).
• git grep -F of the old password on main: this file is the only copy, so after the merge the tree is clean. The file is unchanged on main since the merge base; git merge-tree clean.
• Caller fpga/fly-vivado/startup.sh runs the script in the same container env, so the two variables must be set as deployment secrets.
• Still required from the owner: the password stays in git history -- rotate the AMD account password.