Fix export-context history attribution

[codeforester]

Summary

• Load the shared project-command helpers before export-context sets history attribution.
• Validate the resolved project name, root, and manifest before exporting them into the history environment.
• Strengthen BATS coverage for clean output, complete history metadata, and incomplete resolver results.

Issue

Fixes #1978

Validation

• BASE_BASH_LIBS_DIR="$HOME/work/base-bash-libs/lib/bash" bats cli/bash/commands/basectl/tests/export-context.bats (6 passed)
• shellcheck cli/bash/commands/basectl/subcommands/export_context.sh
• Real basectl export-context base --list-files smoke test with isolated cache; stderr was clean and the history record contained project, project_root, and manifest.
• env -u BASE_HOME TZ=UTC BASE_BASH_LIBS_DIR="$HOME/work/base-bash-libs/lib/bash" BASE_CLI_SOURCE_DIR="$HOME/work/base-cli/lib/python" BASE_CACHE_DIR=/private/tmp/base-1978-full-utc ./bin/base-test (968 Python tests and 871 BATS tests passed)
• git diff --check

Demo Impact

None.

Notes

.ai-context/ is unchanged because this restores the existing documented command and history contracts without changing the public command surface or product architecture.