Drop the top-level vendor/tinycortex submodule in favour of tinymemory's nested copy

[YellowSnnowmann]

Summary

Remove the top-level vendor/tinycortex submodule and resolve tinycortex / tinycortex-api through the copy vendor/tinymemory already vendors (vendor/tinymemory/vendor/tinycortex), so the engine the tests link is by construction the one the prebuilt tinymemory module was built from.

Problem / Context

• TinyCortex left the product build in Route memory tool and query paths through the module seam so tinymemory-core leaves the build #5560; the shipped app reaches memory through the prebuilt tinymemory module over tinymemory-api (cargo tree -e normal -i tinycortex under the product feature set prints nothing).
• The top-level submodule only served the test engine (tinymemory-core + tinymemory-tinycortex in [dev-dependencies], plus the default-OFF rss-bench bin). Those crates name tinycortex / tinycortex-api by version requirement; tinycortex-api is unpublished and crates.io's tinycortex is a stale 0.1.1 — so the root [patch.crates-io] must exist, and its checkout had to be bumped by hand in lockstep with vendor/tinymemory (chore(modules): re-pin tinymemory to v1.15.0 #6049 moved both pointers). A stale top-level pointer silently tests a different engine than the module ships.
• Costs: tinycortex cloned twice per recursive checkout (90 MB tree + 66 MB objects); the tinycortex-tests job runs the engine's own suite on every full Rust CI run (30-minute budget), duplicating tinycortex's own CI; app/src-tauri/Cargo.toml carries two patch entries its lock lists as [[patch.unused]]; AGENTS.md's "patches in both manifests" rule describes the pre-Route memory tool and query paths through the module seam so tinymemory-core leaves the build #5560 graph.
• Recurring confusion: "there are tinycortex mentions in the repo" keeps being read as "the product still uses tinycortex".

Scope

In: gitlink + .gitmodules removal; root patch re-point; shell manifest dead patches + lock entries; tinycortex-tests job in ci-lite.yml / test-reusable.yml; AGENTS.md rule; doc comment / architecture doc.

Out: removing TinyCortex from the test suite ([dev-dependencies], ~100 test files, 26 integration targets) — separate project; the LEGACY_MODULE_ID = "tinycortex" config alias; historical docs/tinycortex-*.md.

Acceptance criteria

• Submodule gone — no vendor/tinycortex gitlink or .gitmodules entry; git submodule status --recursive lists tinycortex only under vendor/tinymemory/.
• Root patch re-pointed — tinycortex / tinycortex-api → vendor/tinymemory/vendor/tinycortex[/api]; root Cargo.lock unchanged; cargo metadata --locked passes for both manifests.
• Product unchanged — cargo tree -e normal --features "$(bash scripts/ci/product-features.sh)" -i tinycortex prints nothing; product cargo check passes.
• Test engine resolves from the nested path — cargo check -p openhuman --lib --tests passes; memory::tree_e2e_tests and memory::sync_pipeline_e2e_tests pass.
• Shell world clean — no tinycortex patch entries in app/src-tauri/Cargo.toml, no [[patch.unused]] for them in its lock; shell all-features cargo tree shows no engine crates.
• CI — tinycortex-tests removed from both workflows with no dangling needs; PR CI Gate map updated.
• Docs — AGENTS.md [patch] rule states root-manifest-only and why.
• Diff coverage ≥ 80% — N/A: no executable code changes (manifests, YAML, comments, docs).

Related

• Implemented by chore(vendor): drop the top-level tinycortex submodule #6061
• Follow-up to Route memory tool and query paths through the module seam so tinymemory-core leaves the build #5560 (engine shed) and chore(modules): re-pin tinymemory to v1.15.0 #6049 (last dual-pointer bump)
• Independent of chore(modules): re-pin the memory module to tinymemory v1.15.2 #6056 (v1.15.2 re-pin; nests the same tinycortex 79131f2)
• Not this issue: migrating the test suite off the in-process engine