read_file/str_replace do not expand leading ~ (resolves under /app/~) #6270

Problem

The dev-mcp read_file / str_replace tools do not expand a leading ~. In resolve_path (crates/buzz-dev-mcp/src/paths.rs), a path is only ever treated as absolute or joined onto the workspace root. A tilde path like ~/.claude/skills/x is not absolute, so it resolves to <workdir>/~/.claude/... (e.g. /app/~/.claude/...), which never exists.

Observed in the PR #6261 named-path benchmark: all five agents initially passed a literal ~ to read_file, which resolved incorrectly under /app/~. Every agent recovered, but the tool ergonomics are wrong — the shell tool expands ~ via bash, so the file tools diverge from it.

Fix

Expand a leading ~ (bare ~ or ~/...) to the user home directory ($HOME, %USERPROFILE% on Windows) at the single chokepoint resolve_path, matching shell semantics. ~user (another user home) is intentionally not handled — it needs a passwd lookup and is out of scope, consistent with the conservative posture for un-mappable MSYS forms.