fix(deps): Treat sibling shadows as unresolved in degenerate single-dir roots

[Xuepoo]

Summary

Aligns bare-target classification with the Unresolved contract introduced in #18: a same-dir namesake is ambiguous, not evidence of locality — in every tree shape.

Two-tier rule (crates/ctxctl/src/deps.rs)

1. Inside a repository (.git ancestor found): a bare-target hit at the project root is decisive — module layouts root there — so it is Local. Files merely beside the analyzed file stay ambiguous (Unresolved). Behavior byte-identical to before.
2. Degenerate standalone case (no .git ancestor anywhere): the fallback root equals the analyzed file's own directory, so a "root" hit is just a same-dir namesake. It now yields Unresolved instead of being guessed Local.

Explicit relative imports (from .x import y, ./x, ../x) bypass both tiers and remain Local — their target path is visible from the file itself. Root selection itself is unchanged (file dir stays a legitimate fallback root for standalone-file usage).

Evidence

Before this change: analyzing main.py (import os) sitting next to an os.py in a directory without any .git ancestor classified os as local — because project_root() fell back to the file's own dir and the root probe then probed right beside it, contradicting the never-a-guess contract at deps.rs:113.

Tests

• Added deps_degenerate_dir_sibling_shadow_is_unresolved_not_local — red first (was local), now green.
• Added deps_degenerate_dir_relative_import_stays_local — explicit relative imports stay local in degenerate trees.
• Verified existing deps_sibling_shadow_of_bare_import_is_unresolved_not_local (.git above analyzed subdir) still passes.
• Updated three language-probe fixtures (deps_go_local_via_project_root_probe, java_outline_symbol_and_deps_local_probe, csharp_deps_local_probe) that relied on the degenerate probe expecting local: added a .git marker so they genuinely exercise tier-1 rooted anchoring, preserving their purpose under the new rule.

Gates

cargo fmt --check, cargo clippy --all-features -- -D warnings, cargo test (13/13 binaries, incl. 98 cli tests) — all green.