chore: retire remaining SpecGit v1 assets and initialize v2

[LeXwDeX]

Why

The v1 acceptance workflow was retired in #66, but this repository still tracked its v1 declaration, policy, generated OpenCode hook, and stale ignore entries. This change completes the reviewed local v1 asset retirement for #68 while keeping the v2 declaration and guidance local to the checkout.

Changes

• Remove the tracked v1 .specgit.yaml, spec_git/policy.yaml, and generated OpenCode hook registration/script.
• Remove the obsolete SpecGit block from .gitignore and the retired workflow/policy entries from .prettierignore.
• Keep the v2 .specgit.yaml on disk under .git/info/exclude; generated AGENTS.md guidance remains a local, unstaged file. The hand-written AGENTS prefix and the business pr-checks.yml and publish.yml workflows are unchanged.

Verification

• Migration preview and exact-digest apply were recorded in the R04 ledger; the final six-path diff was reviewed against Issue chore: retire remaining SpecGit v1 assets and initialize v2 #68.
• PR head f7f8dad23fd2535b019137c3f1dc7cc269ad0510 has the same 80 Git blobs as the index-only source manifest used for local checks.
• Node 26.8.1/npm 12.0.2 on macOS arm64: format, typecheck, 104 tests, package verification, and audit passed; fixed OpenCode source host integration passed 17/17.
• Local OpenCode compatibility targets: plugin/SDK 1.4.3 and latest (resolved to 1.18.32) both passed install, typecheck, build, and ESM import in separate environments.
• Git diff checks passed. The original migration checkout retains its local v2 declaration and generated guidance.

Review and delivery still required

• Require the current PR head's Type Check, Build & Audit, OpenCode 1.4.3 compatibility, OpenCode latest compatibility, Real OpenCode host integration, and aggregate opencode-compatibility checks to pass. These are business acceptance checks; current master is not protected by a required-check rule.
• Review the native request, then verify the exact merge commit, Issue closure, retained local v2 configuration, and absence of active v1 assets. The broader R04 recovery and end-to-end work remains tracked separately.

Closes #68