Ignore detect paths that can't be stat'ed in project detection

[pinin4fjords]

Description

Fixes #14983.

Project detection currently treats an inaccessible detect path as if it exists. This happens because safeExistsSync, which wraps @std/fs’s existsSync, returns true for PermissionDenied.

As a result, a parent directory containing content/ and an inaccessible config/ can incorrectly match Hugo’s config/_default/config.toml detect path, even when that file does not exist. quarto render may then:

• write hugo-md output despite --output doc.html; or
• fail while walking the directory.

This change counts a detect path only when Deno.statSync succeeds. It affects detection only; the input walk still fails on unreadable directories, as intended by #14966.

Scope. Quarto resolves documents using absolute paths, so every directory in the upward walk must be searchable. A _quarto.yml or single-segment detect path is checked in a searchable directory, so a permission error cannot cause a false match there. Among the bundled extensions, only Hugo’s multi-segment detect path (config/_default/config.toml) is affected.

Trade-off. If a Hugo site’s config/ directory cannot be searched, Quarto will no longer detect it as a Hugo site and will render the input as a single file. If only the config file itself is unreadable, the site is still detected (tested on macOS and Linux).

Tests. Added unit and smoke tests that fail without this change and pass with it, on macOS and in CI. The following also pass:

• tests/unit/project
• smoke/project/project-unreadable-dir
• the project check tests from Fix check crash and raw render error on unreadable project dirs #14966
• deno fmt --check
• deno lint

Added one sentence describing this rule to llm-docs/project-context-architecture.md.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes
• updated the appropriate changelog in the PR
• ensured the present test suite passes
• added new tests
• created a separate documentation PR in Quarto's website repo and linked it to this PR (not needed)

AI-assisted PR

• AI tool used: Claude Code
• Codebase grounding: local clone
• Human review: I directed this work, reviewed the investigation, the change and its test results, and verified the behaviour against the Hugo project detection matches a parent folder whose config/ can't be entered, so quarto render can silently write Markdown #14983 reproduction. The change also went through independent review passes.

[posit-snyk-bot]

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