Improve package changelog text controls

[jlitola]

Summary

• Add MCP text controls for pkg_changelog: verbose:true and body_lines for body preview management.
• Improve changelog text readability with ASCII punctuation, normal-contrast body text, and highlighted entry headers.
• Update docs, MCP instructions, smokes, tests, and add a range-focused agentic eval workload.

Verification

• bun test
• bun run build
• bun run smoke:mcp
• bun run smoke:cli
• bun run agent:e2e --agent claude --server local --workload eval/agentic/workloads/package-changelog-range.md --timeout 300
• bun run agent:e2e --agent codex --server local --workload eval/agentic/workloads/package-changelog-range.md --timeout 300

Agentic Eval Notes

• Claude used pkg_changelog with from_version, to_version, and verbose:true, then supplemented with pkg_vulns.
• Codex used pkg_changelog JSON and supplemented with pkg_deps/pkg_vulns.
• Both reported success with no tool or instruction issues.

Follow-up

• pkg_vulns --verbose cannot currently list historical non-affecting advisories because the client query requests advisories(scope: AFFECTED). Backend schema supports NON_AFFECTING and ALL; handle that in a dedicated follow-up PR.