fix(tui): skip project-scope config merge when workspace is home directory

[Lellansin]

Summary

• skip project-scope config merge when the workspace root resolves to the user home directory
• when cwd == $HOME, the project file (~/.deepseek/config.toml) is also the global config file — merge is redundant and the warning is noise
• guard early in merge_project_config via dirs::home_dir() + std::fs::canonicalize

The #417 deny-list correctly refuses dangerous keys at project scope, but the warning fires even when the project file is the same file as the global config.

Tests

• cargo fmt --all -- --check
• cargo check -p deepseek-tui --locked
• cargo clippy -p deepseek-tui --all-targets --all-features --locked -- -D warnings
• cargo test -p deepseek-tui -- merge_project_config project_overlay --locked (15/15 passed)
• git diff --check

Reproduce (main)

tmux -S /tmp/claude.sock new -d -s repro -n shell
tmux -S /tmp/claude.sock send-keys -t repro:1.1 -- \
  'cd ~ && deepseek-tui 2>/tmp/repro.log' Enter
sleep 4
tmux -S /tmp/claude.sock send-keys -t repro:1.1 C-c  # first Ctrl+C
sleep 1
tmux -S /tmp/claude.sock send-keys -t repro:1.1 C-c  # second Ctrl+C
sleep 2
cat /tmp/repro.log
# Output: warning: project-scope config key `api_key` is ignored …

Verify (this branch)

Same steps produce no project-scope config key … ignored warning.

Environment: macOS, ~/.deepseek/config.toml contains api_key. TUI started from ~.

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[Hmbown]

This PR was opened before the v0.8.41 rebrand and is now stale. Feel free to rebase onto current main and reopen. 鲸鱼兄弟们等你 🐋

[Lellansin]

Rebased and force-pushed the branch to current main. GitHub refuses to reopen this PR because the branch was force-pushed/recreated, so I opened replacement PR #2055.