chore: harden Varlock runtime contract

[WalksWithASwagger]

Summary

• extend the committed Varlock schema for Replicate and agentic runner settings
• replace the remaining documented direct .env runtime path with varlock run --inject vars --
• add agent-safe secret handling and a redacted injection smoke check

Verification

• npm run docs:check
• npm run public:check
• npm run pack:check
• git diff --check
• gitleaks dir --redact --no-banner .

Remaining environment check

• Varlock is not installed in this isolated worktree environment, so varlock load --agent --show-all and the injection smoke check remain for a Varlock-enabled environment.
• The default Python test environment lacks project dependencies (mcp, jsonschema), so the full test suite was not used as a gate here.

Refs #310

[github-actions]

Agentic PR Review

Verdict: needs-human

Checks:

• Linked issue via Closes #... or Refs #...: yes
• Issue acceptance criteria found: 5
• PR verification section present: yes
• PR acceptance self-check present: no
• Diff within v1 limits: yes

This PR is not blocked from human review, but the agentic gate is not satisfied.

[WalksWithASwagger]

Superseded by fully verified PR #312, which covers the same issue with passing Varlock audit/injection/staged scan, the full 511-test verification suite, and green GitHub checks.