license: move Base to Apache-2.0

[codeforester]

Summary

• move Base's current license from AGPL-3.0-or-later to Apache-2.0 prospectively
  starting with v1.9.0, while preserving historical release boundaries
• make basectl repo init generate Apache-2.0 repositories by default and
  remove the obsolete AGPL/copyright-holder generator path
• update README, contributor guidance, product assessment, baseline docs,
  command reference, help, and Bash/Zsh completions

Adoption Rationale

Base is a local CLI and workspace control plane, so Apache-2.0 removes the
strong-copyleft review friction without imposing a network-reciprocity contract
that is central to the product. Apache-2.0 also aligns Base with the existing
base-cli and base-bash-libs repositories.

Existing releases remain unchanged: versions through v1.0.0 remain MIT, and
v1.0.1 through v1.8.0 remain AGPL-3.0-or-later.

Validation

• git diff --check
• focused repository, completion, help, and command-reference BATS: 156 passed
• TZ=UTC BASE_BASH_LIBS_DIR=... BASE_CLI_SOURCE_DIR=... env -u BASE_HOME ./bin/base-test: 960 passed, 1 skipped

Fixes #1959