Remove strict mode from repository validation scripts

[codeforester]

Problem

STANDARDS.md inherits Base's Bash standards. Base section 3.3 says Base-owned executable shell files must not use set -e, set -u, or pipefail; only GitHub Actions run: blocks are a narrow exception.

At main commit a107134, both executable repository validation entrypoints use set -e:

• tests/validate.sh
• tests/lint-warnings.sh

That makes the canonical local gate depend on implicit shell exit behavior instead of making each validation failure explicit.

Scope

• Remove set -e from both validation scripts.
• Add explicit failure handling and useful diagnostics for every required command, pipeline, loop, example, and delegated test.
• Keep workflow-local strict mode in .github/workflows/ unchanged.
• Add a lightweight validation guard that prevents strict mode from returning to production/validation shell entrypoints while allowing intentional BATS fixtures.

Acceptance Criteria

• The validation scripts contain no strict-mode enablement.
• A failing ShellCheck, BATS suite, example, or delegated warning lint exits nonzero at the correct step.
• Successful output remains concise and the canonical command remains ./tests/validate.sh.

Validation

• tests/lint-warnings.sh
• ./tests/validate.sh
• targeted negative smoke tests for each delegated stage
• git diff --check