Fix bugs, replace unreliable git validation

[petetomasik]

• plugin.yml: Fixed plugin name (Commit Branch → Branch Commit), description typo, removed invalid required: mandatory field, added enum/default constraints to mode
• lib/plugin.bash: Fixed PLUGIN_PREFIX from COMMIT_BRANCH to BRANCH_COMMIT, removed 4 unused list functions
• hooks/post-checkout: Added set -euo pipefail, quoted all variables (shell injection fix), sourced lib/plugin.bash via DIR= pattern, added BUILDKITE_COMMIT validation, mode validation, and stderr output. Replaced unreliable git name-rev with git merge-base --is-ancestor for shallow/partial clone compatibility. Handles fetch failures gracefully
  with distinct error messages
• tests: Deleted broken tests/command.bats (wrong hook, wrong env vars, no stubs), created tests/post-checkout.bats with 10 passing tests covering both modes, early exits, missing env vars, shallow/non-shallow repos, branch mismatch, and fetch failures
• .buildkite/pipeline.yml: Fixed linter ID from template to branch-commit, shellcheck glob from lib/** to lib/*.bash
• README.md: Fixed invalid YAML example, documented mode as optional with strict default, added shallow clone behavior note, replaced placeholder content with actual developing/contributing sections

[pzeballos]

LGTM

I would suggest considering splitting the logic (like, validate_env, fetch_branch, check_commit_ancestry), and the log output could be standardized with a prefix for clarity.

For example something like:

PLUGIN_LOG_PREFIX="[branch-commit-plugin]"

log_info() {
  echo "$PLUGIN_LOG_PREFIX INFO: $1" >&2
}

log_warn() {
  echo "$PLUGIN_LOG_PREFIX WARNING: $1" >&2
}

log_error() {
  echo "$PLUGIN_LOG_PREFIX ERROR: $1" >&2
}

validate_env() {
  if [[ "${BUILDKITE_SOURCE:-}" != "ui" ]]; then
    exit 0
  fi
  if [[ -z "${BUILDKITE_BRANCH:-}" ]]; then
    log_error "BUILDKITE_BRANCH environment variable is not set."
    exit 1
  fi
  if [[ -z "${BUILDKITE_COMMIT:-}" ]]; then
    log_error "BUILDKITE_COMMIT environment variable is not set."
    exit 1
  fi
}

validate_mode() {
  local mode="$1"
  if [[ "$mode" != "warn" && "$mode" != "strict" ]]; then
    log_error "Invalid mode '$mode'. Must be 'warn' or 'strict'."
    exit 1
  fi
}

...

But it's not a blocker; it's just to reduce complexity, make the script safer (by reducing repetition) and easier to extend