fix(tool-sandbox): skip missing fs_write_file grants instead of denying

[kipz]

Linked Issue

Closes #1450

Summary

A per-command fs_write_file grant for a missing path denied the whole
sandboxed command, unlike the sibling fs_read/fs_write/fs_read_file
grants which already skip missing paths. Adds a shared
add_optional_write_file helper (in tool-sandbox/mod.rs, alongside the
existing agent_can_write/lexically_normalize shared helpers) matching
that existing pattern, called from both the macOS and Linux platform
implementations instead of each defining its own copy.

Agent Disclosure

Authored by an AI coding agent on behalf of @kipz. Consulted
tool-sandbox/platform/{macos,linux}.rs (add_policy_fs and its
add_optional_* siblings) and capability.rs/capability_ext.rs to
confirm this path lacks the missing-file tolerance the top-level session
sandbox already has. Disclosed on the linked issue before opening this PR.

Test Plan

• make ci — clean.
• Added add_optional_write_file_tests (missing path is skipped, existing
  path is granted) — both pass.
• Rebuilt a downstream consumer against this branch and re-ran the issue's
  repro: denial is gone, command proceeds past sandbox construction.

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 (Required for AI/Automated PRs)

• 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	+50
Lines removed	-2
Total changed	52
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.}

[lukehinds]

all good thanks @kipz !