✨ feat: prefer latest tag on current branch when resolving tags

[AliRezaTaleghani]

Improve tag discovery and commit range computation by introducing a
helper that finds the latest tag on the current branch and using it
consistently across functions.

• Add getLatestTagOnBranch(prefix) to obtain current branch and
  run git describe --match against that branch. This
  centralizes tag lookup and trims whitespace before returning.
• Replace ad-hoc git describe invocations in last-release lookup
  with getLatestTagOnBranch to avoid incorrect tag resolution and
  simplify error handling.
• Use the canonical string (already trimmed by the helper) when
  trimming the prefix and when constructing git log ranges.
• Handle empty tag results consistently (check for empty string
  instead of byte-length) and preserve fallbacks to analyzing all
  commits when no prior tag is found.

These changes make tag lookup branch-aware, reduce duplicated git
invocation logic, and prevent subtle bugs from trimming or handling
raw command output.