feat(shell): implement getcwd/chdir syscalls for shell navigation

[ryanbreen]

Summary

• Add per-process current working directory (cwd) tracking with full POSIX-compliant getcwd and chdir syscalls
• Enable shell navigation with cd and pwd commands
• Fix ls to list current directory by default instead of root

Changes

Kernel

• Add cwd field to Process struct (initialized to "/")
• Implement sys_getcwd (syscall 79) - returns cwd in user buffer with POSIX pointer return
• Implement sys_chdir (syscall 80) - validates path exists and is directory
• Add relative path resolution to sys_open using process cwd
• Fork inherits cwd from parent process
• Handle devfs paths correctly (ENOTDIR for device files)

Shell

• Add cd and pwd built-in commands to init_shell
• Update ls_cmd to default to "." (cwd) instead of "/" (root)

Tests

• New cwd_test.rs with 7 comprehensive tests
• 8 new boot stages for CWD validation
• Test coverage: initial cwd, chdir success, ENOENT, ENOTDIR, relative paths, fork inheritance, getcwd error handling

Test plan

• All 210 boot stages pass
• Interactive shell cd /test && ls works correctly
• pwd shows correct directory after cd
• Error handling validated (ENOENT, ENOTDIR, EINVAL, ERANGE)

🤖 Generated with Claude Code

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Note

Introduces filesystem navigation capabilities and wires them through kernel, libc, shell, and tests.

• Kernel: add Process.cwd, implement sys_getcwd (79) and sys_chdir (80), wire in dispatcher/handler, add ERANGE, make sys_open resolve relative paths via cwd with normalize_path, and ensure fork inherits cwd in all fork paths
• Userspace/lib: add process::getcwd and process::chdir wrappers; shell adds built-ins cd and pwd; ls_cmd defaults to "." when no path is given
• Tests/build: new cwd_test binary with 7 checks (initial /, chdir success/ENOENT/ENOTDIR, relative .., fork inheritance, getcwd errors) and 8 new boot markers; kernel runs test_cwd; build script and Cargo wired
• Docs: add docs/planning/SHELL_AND_APPS_ROADMAP.md outlining roadmap for shell and third‑party apps

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Empty path in sys_open resolves to cwd instead of ENOENT

Medium Severity

The new relative path handling in sys_open lacks a check for empty paths. When open("") is called, the empty string doesn't start with '/', so it enters the relative path resolution branch. This causes an empty path to resolve to the current working directory (e.g., "/test/" normalizes to "/test"), whereas POSIX requires open("") to fail with ENOENT. The sys_chdir function correctly handles this case with an explicit empty path check, but sys_open is missing the equivalent validation.