chore: remove arghda-core/ subtree + cross-refs (extracted to standalone repo per #159)

[hyperpolymath]

Summary

Per owner directive 2026-05-30, `arghda-core` is not close enough to echo-types' logic to justify being in-tree. It's been extracted to its own repo at https://github.com/hyperpolymath/arghda-core with full git history preserved via `git subtree split --prefix=arghda-core HEAD`.

This PR completes the move on the echo-types side: delete the subtree + the spec doc + update all 8 cross-references.

Counterpart on the new repo

Already landed on https://github.com/hyperpolymath/arghda-core/main:

• 4 commits of historical arghda-core history preserved (subtree-split from echo-types)
• Bootstrap: standalone-repo plumbing (Cargo.toml repo URL fixed, .gitignore, README rewritten, smoke tests rewritten from echo-types-relative integration to self-contained fixtures)
• CI: self-contained Rust workflow (cargo fmt + build + test + clippy -D warnings) — verified green at https://github.com/hyperpolymath/arghda-core/actions/runs/26684219505
• `docs/arghda-spec.adoc` imported from echo-types verbatim

What this PR does

File	Change
`arghda-core/` (12 files, ~1450 LOC)	Delete subtree
`docs/arghda-spec.adoc`	Delete (moved to new repo)
`.gitignore`	Drop `arghda-core/target/`
`0-AI-MANIFEST.a2ml`	Drop `arghda-core/` from agent-may allowlist
`QUICKSTART-DEV.adoc`	Drop `rust (for arghda-core/)` dev-dep
`TOPOLOGY.adoc`	Drop `arghda-core/` tree-diagram line
`CLAUDE.md` ecosystem block	Point at new repo URL + record 2026-05-30 extraction
`flake.nix`	Drop `pkgs.{rustc,cargo,rustfmt,clippy}` from devShell; PINNING-DECISION block records the extraction
`docs/echo-types/MAP.adoc` §Tooling	Replace in-tree ArghDA block with external pointer
`contractiles/Mustfile.a2ml`	Drop `rust-compiles` must; narrow `spdx-headers` to Agda-only
`contractiles/Dustfile.a2ml`	Drop `rm -rf arghda-core/target` from `clean-build`
`audits/assail-classifications.a2ml`	Drop the now-stale UnboundedAllocation entry for the orphan_module.rs file; rewrite metadata notes; keep flake.nix SupplyChain entry

What this PR deliberately leaves

Four `arghda` mentions remain in the tree — all extraction-record breadcrumbs by design:

• `flake.nix` PINNING-DECISION comment — historical context
• `CLAUDE.md` ecosystem-context block — points future agents at the new repo
• `audits/assail-classifications.a2ml` metadata — records why the orphan_module.rs entry left
• `docs/echo-types/MAP.adoc` §Tooling — external pointer

Test plan

• Agda suite unaffected — no Agda files moved. Verified clean against post-audits: assail-classifications.a2ml — Track-C suppressions (echo-types#113) #158 main earlier this session (All.agda + Smoke.agda + characteristic + examples all exit 0 under `--safe --without-K`; kernel-guard + check-guardrails both pass).
• Narrowed `spdx-headers` must rule still passes (Agda-only scope).
• New repo's CI green: https://github.com/hyperpolymath/arghda-core/actions/runs/26684219505

Refs

• Closes arghda-core/ has no CI workflow — Rust crate is in-tree but untested #159
• New repo: https://github.com/hyperpolymath/arghda-core
• Spec moved to: `arghda-core/docs/arghda-spec.adoc` in the new repo

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 58 issues detected

Severity	Count
🔴 Critical	17
🟠 High	22
🟡 Medium	19

⚠️ Action Required: Critical security issues found!

View findings

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  {
    "reason": "Action rpolymath/standards/.github/workflows/governance-reusable.yml@main\n needs attention",
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]

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