[imfing]

Summary

• Adds diffs branch -i / --interactive: a fuzzy-filterable picker over local and remote-tracking branches (type to filter, Enter to select).
• Without a base, the picker chooses the base: the checked-out branch is left out and the inferred base (PR base, default branch, or main/master) is preselected.
• With a base (diffs branch main -i), the picker chooses the head instead, with the checked-out branch preselected. -i conflicts with an explicit head.
• Esc, q, or Ctrl-C exit 1 without output; a non-terminal stdin/stderr fails with a hint to pass the base explicitly.
• Picker colors follow the existing NO_COLOR/TERM=dumb handling.

Uses dialoguer (fuzzy-select feature only) and console for the terminal UI. Help text and README updated.

Test plan

• cargo clippy --all-targets -- -D warnings, cargo test (new picker_list unit test)
• Drove the picker through a pty: filtering rele + Enter started the server with base=release/1.0; Esc exited 1 quietly; diffs branch main -i with hot + Enter started with head=hotfix
• diffs branch -i </dev/null fails with the terminal hint; diffs branch main feature -i is rejected by clap

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