[Chore] Bump Go version to latest, adopt golangci-lint v2, add Docker-based build, and clean up AGENTS.md/skills

## Summary
A grab-bag of repository hygiene chores for the BanyanDB - 0.12.0 cycle.

## Sub-tasks
1. **Bump Go version to latest**
- Update `go.mod` `go` directive.
- Update any `-dev`/`-test` images and CI workflows.
- Verify `make build` / `make test` on the new toolchain.
- Document the new minimum supported Go version in `README.md` / `CONTRIBUTING.md`.
2. **Use golangci-lint v2**
- Bump the `.golangci.yml` / `revive.toml` / lint config to the v2 schema.
- Update CI workflows (`.github/workflows/*.yml`) and the `run-ci-local` skill if pinned.
- Fix any new lints surfaced by v2.
3. **Introduce a Docker-based build system for consistent license-file generation across OSes**
- Add a `build/Dockerfile` (or extend existing) that produces a reproducible build environment.
- License generation: ensure the produced NOTICE / LICENSE-3rd-party output is byte-identical regardless of host OS (line endings, sort order, timestamps).
- Wire it into `make` targets and CI so contributors can run `make license-check` locally with the same guarantees.
- Document usage in `CONTRIBUTING.md` / `README.md`.
4. **Clean up AGENTS.md and skills**
- Audit `AGENTS.md`, `CLAUDE.md`, and the entries under `skills/` for stale / redundant / conflicting guidance.
- Remove duplication, fix references to renamed paths, and align with current Go / repo conventions.
- Update the index files / README that list available skills.

## Acceptance Criteria
- Each sub-task has a checkbox / PR reference below as it lands.
- CI green on the new toolchain and with golangci-lint v2.
- License generation is reproducible across Linux/macOS/Windows hosts.
- AGENTS.md and skills directory are consistent and current.

## Sub-task tracker
- [ ] (1) Bump Go version — PR #___
- [ ] (2) golangci-lint v2 — PR #___
- [x] (3) Docker-based build for license generation — moved to #13996
- [ ] (4) Clean up AGENTS.md and skills — PR #___