fix(cli): reject upstream proxy with block net

[dpersek]

Linked Issue

Related to #1233

Summary

This PR makes hard network blocking reject upstream proxy configuration consistently instead of allowing proxy-mode semantics to win silently.

• reject --block-net / profile network.block when --upstream-proxy or profile upstream_proxy is present
• share the upstream-proxy presence check with upstream-bypass validation
• run the same proxy-conflict validation for nono shell dry-run and launch paths before proxy launch options are built
• update a stale runtime comment that still described profile network.block as yielding to proxy config

Agent Disclosure

This PR was prepared by an AI coding agent working with Dustin.

Before implementation, Dustin posted the repository-required issue-disclosure/intent/approach comment on #1233. I reviewed AGENTS.md, CLAUDE.md, CONTRIBUTING.md, SECURITY.md, .github/pull_request_template.md, crates/nono-cli/data/profile-authoring-guide.md, docs/cli/features/networking.mdx, and the affected Rust modules. This PR stays within the disclosed scope and does not reuse or adapt external code.

Repository-rule checks:

• no new production unwrap() or expect() calls
• new conflict errors use NonoError::ConfigParse
• tests cover the new security-sensitive configuration conflict
• the profile authoring guide already documents that network.block: true cannot be combined with proxy settings, so no docs change was needed

Test Plan

• cargo test -p nono-cli shell_dry_run_rejects_block_net_with_upstream_proxy
• cargo test -p nono-cli upstream_proxy
• cargo test -p nono-cli block_net
• make fmt-check
• make clippy
• make test-cli

Notes:

• make test-cli was run outside the local Codex sandbox after a sandboxed run failed on local supervisor/attach socket binds with Operation not permitted.
• Full make ci was not run locally; make test-cli, make clippy, and make fmt-check passed.

Checklist

• An issue exists and is linked above
• All commits are signed-off, using DCO
• All new code follows the project's coding standards (CLAUDE.md) and is covered by tests
• Public-facing changes are paired with documentation updates
• Release note has been added to CHANGELOG.md if needed

Agent Compliance Check

• I am not prohibited from contributing under this policy
• An issue already exists
• I disclosed that I am an agent in the issue discussion
• I described my intent and approach in the issue discussion
• I reviewed repository coding and security rules for the affected area
• I provided required attribution for reused or adapted code
• I did not use forbidden patterns such as unwrap/expect
• I used NonoError where required
• I validated and canonicalized all relevant paths
• This PR matches the approved or disclosed issue scope

[github-actions]

PR Review Summary

Size

Metric	Value
Lines added	+176
Lines removed	-11
Total changed	187
Classification	Medium (50–300 lines)

Affected crates

• crates/nono-cli — CLI changes. Verify argument parsing, flag documentation, and UX behaviour across supported platforms.

Blast radius — Contained

This PR touches: source code

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_{Updated automatically on each push to this PR.}

[nogent-nolabs-ai]

nogent code review

No blocking issues; 0 suggestions.

Findings: none flagged in scope.

_{Automated code + security review. CI already covers clippy, rustfmt, tests, cargo-audit and commit-lint.}