chore(dev): add quantmind-dev skill and align contributor instructions

[keli-wen]

Description

Adds a contributor-focused instruction layer for coding agents working on the QuantMind codebase. Closes #93.

• AGENTS.md / CLAUDE.md: rewritten as thin, aligned entry points — module map with dependency roles, canonical setup/verify commands, stable architecture constraints, test/example requirements, communication conventions, and routing to the quantmind-dev skill. Migration-state tables and PR-by-PR roadmap removed (temporary repository state; history lives in git log and Tracking: Migrate Agent layer to OpenAI Agents SDK #71).
• quantmind-dev skill: mirrored identically under .agents/skills/ and .claude/skills/ with progressive disclosure (SKILL.md routes; only the matching reference loads):
  • references/commit.md — English Conventional Commits, meaningful scopes matching repo history, one logical change per commit, inspect staged diff first, never bypass hooks without explicit authorization.
  • references/pull-request.md — small focused PRs, Conventional Commit titles, English bodies, issue references, verification statement, post-merge branch hygiene.
  • references/develop-components.md — general development loop (find the nearest existing pattern first), import-linter dependency contracts as a who-may-import-whom table, per-module guidance (knowledge/configs/preprocess/flows/mind/utils), unit-test and example requirements, verification steps.
• CONTRIBUTING.md: stale pre-migration content removed (deleted modules, removed scripts, CLI tips, ABC-based patterns); now points to scripts/verify.sh and the canonical workflow.

No runtime code, CLI, configuration schema, or dependency introduced.

Verification: bash scripts/verify.sh green — all 5 steps pass (ruff format, ruff check, basedpyright, lint-imports, pytest: 233 passed, 89.48% branch coverage). Skill mirrors verified byte-identical with diff -r.

Checklist

• The PR title starts with $CATEGORY(xx): xxx
• Related issue is referred in this PR
• The markdown and latex are rendered correctly.
• The code in PR is well-documented.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR.

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