fix: harden artifact integrity determinism

[RyshMan]

Problem

• artifact verification paths could accept ambiguous duplicate ZIP entry names
• MCP trust snapshots could silently tolerate duplicate normalized identities instead of failing closed
• Python run capture helpers were synthesizing digest-bearing fields outside the Go-authoritative normalization path

Changes

• reject duplicate ZIP entry names across runpack, pack, and guard verification flows and add CLI/coverage tests
• fail closed on invalid duplicate MCP trust snapshot identities and distinguish invalid vs unavailable snapshot states
• move Python run-capture digest completion into gait run record normalization, tighten JSON serialization, and document the contract
• wire the new regression coverage into ci.yml and pr-fast.yml

Validation

• gait doctor --json
• make prepush-full
• pre-push hook make prepush

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: c90958e0d3

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".