Refiner: Replace custom tilde expansion in doctor orchestrator with expand_user_path

[kunkunGames]

What changed: Replaced custom manual tilde expansion in src/cli/doctor/orchestrator.rs with crate::runtime_layout::expand_user_path.

Why: PR #212 replaced duplicate tilde expansion with expand_user_path. As part of Refiner, this replaces custom tilde expansion while strictly guarding with starts_with('~') to preserve byte-for-byte equivalence for non-tilde paths.

WorkFingerprint:
Agent: Refiner
Category: refactor
Primary files: src/cli/doctor/orchestrator.rs
Invariant protected: behavior-preserving, fallback behavior for non-tilde paths stays byte-for-byte equivalent.
Public API impact: None
Docs impact: None
Verification plan: cargo check --all-targets, git diff --check
Related PRs/issues: None

Duplicate/overlap check: Verified using gh/git no overlapping PRs before starting.

Verification commands and results:
cargo check --all-targets (Passed)
git diff --check (Passed)

Skipped checks with reasons: No docs generated as no docs changed.

Risk: Low. Behavior is strictly preserved via the guard check.

Rollback notes: Revert the commit.

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PR created automatically by Jules for task 7826566335044703753 started by @kunkunGames

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