fix(up): skip unresolvable paths instead of failing the whole upload

[codyde]

Problem

A single dangling symlink anywhere in the tree aborts railway up during indexing with No such file or directory, and no deployment is created. An exact-path .railwayignore rule cannot exclude it: follow_links(true) makes the walker stat the symlink target before the ignore rules are consulted for that entry, so the error is raised before matching happens.

Closes #1023.

Resolution

• Treat a walk entry whose error is NotFound as skippable — a broken link, or a file removed mid-walk, has no contents to upload — and report it via reporter::warn, so it is structured under --json.
• Any other error still fails the deploy; a permission or I/O error would mean a silently incomplete tarball.
• Validate the root path up front. It reaches the walker as the same NotFound, and none of the three callers check it, so skipping it would turn a typo'd path into an empty but successful deploy.

Not follow_links(false): that regresses symlinked directories, which is what #296 fixed.

Verification

Real deploys, identical tree (backend/AGENTS.md dangling + /backend/AGENTS.md in .railwayignore):

	before (5.30.4)	after
exit code	1	0
deployment	none created	SUCCESS
--json stdout	—	valid JSON

Deployed app serves correctly, so real files still ship. Covered by 7 tests including symlinked-dir traversal, symlink loop stays fatal, and the missing-root and empty-dir/single-file boundaries.