update-design skill: hermetic http_archive workflow

[mguthaus]

Brings .claude/skills/update-design/SKILL.md in line with the repo's hermetic RTL model. The Upstream Audit section still described git submodules and setup.sh tool pins, which no longer exist.

• Audit step 1: audits design source pins as http_archive stanzas in MODULE.bazel, using the gh api .../compare commits-behind check. Adds a "patches at risk" note. Uses ternip, plus its shared @basejump_stl_src archive, as the worked example.
• Audit step 2: audits the converter-tool pins (pip hubs, Maven/Chisel, sv2v, yosys-slang) that replaced setup.sh.
• Audit step 3: reports how far the bazel-orfs pin is behind but hands the bump to /upgrade-tools. The old text described a git_override for ORFS that no longer exists.
• D (FakeRAM): generates macros with bsg_fakeram via tools/regenerate_sram.sh and diff_sram_size.sh instead of hand-editing templates. It now covers:
  • the camelCase snap-key rule for asap7 cfgs
  • the check that no wd_in pin is DIRECTION OUTPUT in the LEF
  • the flip-flop stub fallback for memories too small for the generator
  • swapping in the macro via a declarative patch or by excluding the behavioral module from :rtl
• E (porting): points at /port-design and adds gt2n clock-period guidance, derived from the current results.html.
• Webpage refresh step: the platform count now includes gt2n.

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