fix(storage): reject non-directory paths in project add

[riba2534]

Summary

runtime-host project add did not validate that the supplied path is a directory. A regular file inside a Git repository returned Project catalog mutation outcome is unknown and drained the Host; outside Git the same file was accepted as an unavailable folder project.

resolveProjectLocation already realpath'd the input but never checked stat().isDirectory() before repository discovery. Inside a Git worktree that reached git -C <file> rev-parse ..., whose numeric failure is not an invalid-path errno, so the coordinator called requestDrain(). Reject non-directories with TypeError so register/add treat them as invalid input and the Host stays up.

Fixes #5220

Verification

export PATH="/home/box/.local/node-v22.19.0/bin:$PATH"
npm ci --ignore-scripts
node scripts/apply-dependency-patches.mjs
npm --workspace @maka/core run build
npm --workspace @maka/storage run build
cd packages/storage && node --test dist/__tests__/project-catalog.test.js

Result: 23/23 pass, including new test a regular file is rejected as a project location (file outside Git, file inside a Git repo, and catalog.register of that file).

Did not run full workspace npm test or the CLI Host reproduction from the issue (build surface limited to @maka/core + @maka/storage).

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Cursor / Grok assisted the surgical resolveProjectLocation check and the unit test; commit author/committer is riba2534 only.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[me2seeks]

I found no blocking correctness issue at f708b6e46b46d401c36423e85e02277f7c749d49.

The new check is placed at the right authority boundary: after realpath establishes the canonical target and before Git discovery can run with a non-directory -C path. Directory targets and symlinks resolving to directories keep their existing behavior, while regular files and other non-directory filesystem nodes become typed invalid input. The Host coordinator already maps that TypeError to invalid_request, so the mutation does not enter the uncertain-commit drain path.

Validation on the exact head: clean install and test build; 23/23 project-catalog tests; Biome and whitespace checks; and an isolated real Runtime Host CLI reproduction. Both a file inside a valid Git repository and a file outside Git were rejected with Project catalog input is invalid; project list succeeded after each rejection and the same Host process remained alive.

中文说明

我在精确提交 f708b6e46b46d401c36423e85e02277f7c749d49 上没有发现阻塞问题。

检查位置合理：先用 realpath 得到真实目标，再在 Git 探测前确认它确实是目录。因此真实目录及指向目录的符号链接不受影响，普通文件和其他非目录节点会作为无效输入返回，也不会再进入“提交结果未知”并关闭 Host 的路径。

本地干净构建和 project-catalog 测试 23/23 通过。我还启动了隔离的真实 Runtime Host：Git 仓库内文件和仓库外文件均被拒绝；每次拒绝后 project list 都成功，同一个 Host 进程仍然存活。