feat: add HCP run-id input source

[dwizzle204]

Summary

• Add native HCP Terraform run-id input via --hcp-run-id <id> so tfplan2md can fetch plan JSON directly from HCP/TFE.
• Keep the existing parse/render pipeline unchanged by introducing this strictly as a new input source.
• Add focused docs and tests for authentication, endpoint validation, conflict handling, and API error paths.

What Changed

• CLI / wiring

  • Added --hcp-run-id parsing and input-mode conflict validation.
  • Routed HCP mode through ProgramEntry input selection logic.

• New HCP input adapter

  • Added HcpTerraformPlanInput to:
    • require TFE_TOKEN
    • optionally use TFE_ADDRESS (default https://app.terraform.io)
    • resolve run → plan relationship
    • fetch plan JSON output
    • return actionable errors for auth/HTTP/payload failures

• Tests

  • Added mocked HTTP tests for:
    • successful run-id flow
    • missing token
    • non-success HTTP responses
    • malformed/unexpected payloads
    • HTTPS validation for TFE_ADDRESS

• Docs / examples

  • Updated CLI/help docs for HCP mode and env vars.
  • Added example usage: examples/hcp-run-id/README.md.

Notes

• This PR intentionally does not change markdown rendering behavior.
• Branch history was squashed to a single commit for clean review.

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