docs: add Antigravity config and fix README drift

[ProfRandom92]

Summary

Adds inert, repo-local Antigravity workspace configuration and fixes README command-surface drift.

Changes

• Added .antigravity/ bootstrap, START_HERE, skill inventory, settings, hook/plugin examples.
• Added docs/context/ ledgers and context capsule.
• Fixed README drift:
  • added spark-evidence-demo
  • added spark-evidence-validate
  • replaced hardcoded sandbox file:/// links with relative links
  • marked placeholder command groups as scaffolded

Safety

• No Rust changes.
• No active hooks.
• No active plugins.
• No MCP config.
• No secrets.
• No provider keys.
• No production, legal, compliance, or official SPARK claims.

[gemini-code-assist]

Code Review

This pull request introduces the Antigravity workspace configuration, rules, and guidelines, including several inert configuration templates, hooks, and plugins. It also updates the README.md to document new sparkctl commands and relative paths, and adds detailed context ledgers under docs/context/. The review feedback highlights an inconsistency in the README.md where the Reviewer Quickstart section still references placeholder commands instead of the newly implemented ones.

Important

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[gemini-code-assist]

The Reviewer Quickstart section (immediately following this line) instructs reviewers to run cargo run --bin agy-ct -- run and cargo run --bin agy-ct -- benchmark. However, according to the command mapping table above (line 98), these commands are currently scaffolded placeholders and are not yet implemented. This inconsistency can cause confusion or execution failures for reviewers. Please update this section to refer to the actually implemented commands, such as cargo run --bin sparkctl -- spark-evidence-demo and cargo run --bin sparkctl -- spark-evidence-validate.