fix(security): restrict tokenless dev mode to loopback

[danyalahmed1995]

What

Restrict tokenless development mode to loopback interfaces by default.

When CMCP_DEV_MODE=1 and CMCP_BEARER_TOKEN is unset, the runtime now defaults to 127.0.0.1:8443 and rejects wildcard or non-loopback bind addresses. Authenticated deployments may continue using non-loopback addresses.

Why

Closes #406.

Tokenless development mode previously inherited the default 0.0.0.0:8443 bind, which could unintentionally expose the unauthenticated development gateway to LAN, container, or cloud networks.

This is configuration hardening for development mode, not a production authentication bypass.

Security impact

Reduces the network exposure of tokenless development deployments.

The change:

• allows tokenless binds only on explicit loopback hosts such as 127.0.0.1, localhost, and ::1;
• rejects wildcard, LAN, public, and non-loopback hostname binds without a bearer token;
• preserves non-loopback binds when CMCP_BEARER_TOKEN is configured;
• does not modify authentication middleware, TEE boundaries, signing, audit-chain integrity, capability tokens, or trust-score inputs.

The CLI now also uses the shared validated listen-address parser, including bracketed and unbracketed IPv6 handling.

Test plan

• pytest passes
• ruff check passes
• mypy passes
• Manual test performed (describe steps below if applicable)

Validation completed:

• Focused configuration and startup tests: 49 passed
• Full test suite: 877 passed, 5 skipped
• Ruff passes for all Python files changed by this PR
• python -m compileall -q src tests passes

Repository-wide ruff check . reports pre-existing findings in unrelated scripts and experiments. Repository-wide mypy reports four pre-existing cryptography typing errors in src/cmcp_verify/tdx.py and src/cmcp_verify/sev_snp.py; neither file is modified by this PR.

DCO sign-off

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[github-actions]

🔴 Contributor Check: HIGH

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Codecov Report

❌ Patch coverage is 78.37838% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/cmcp_runtime/config.py	78.37%	8 Missing ⚠️

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[imran-siddique]

Thanks @danyalahmed1995 — merged as bc8b624 on main (via #418), with your authorship preserved. I rebased it onto current main to resolve a conflict in docs/quickstart.md (rewritten in #417); the code, docs, and your tests are unchanged and the full suite is green. Appreciate the hardening.