fix(release-check): reject empty artifact values

[pentaxis93]

Summary

• Rejects supplied empty --agentd-bin and --container-image values during release-check option parsing.
• Preserves source-only release validation when artifact options are omitted.
• Records the release-operator-visible behavior change in the changelog.

Changes

• Adds parse-time non-empty checks for release artifact option values.
• Adds regression coverage for both-empty values and mixed empty/non-empty artifact validation arguments.
• Updates CHANGELOG.md under Unreleased / Fixed.

GitHub Issue(s)

Closes #135

Test plan

• RED: cargo test -p agentd --test release_check release_rejects_empty -- --nocapture failed before the parser change because the empty-value cases unexpectedly succeeded.
• GREEN: cargo test -p agentd --test release_check release_rejects_empty -- --nocapture
• cargo test -p agentd --test release_check
• ./scripts/release-check release v0.1.2-rc.3 --agentd-bin "" fails with --agentd-bin requires a non-empty path
• ./scripts/release-check release v0.1.2-rc.3 --container-image "" fails with --container-image requires a non-empty image
• ./scripts/release-check release v0.1.2-rc.3
• ./scripts/release-check metadata
• cargo fmt --check
• git diff --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo build --workspace
• cargo test --workspace

Documentation

• Updated CHANGELOG.md for the user-visible release-check behavior change.
• Reviewed RELEASING.md, README.md, ARCHITECTURE.md, and AGENTS.md; no drifted claims found for this parser hardening.