[feat] Support changing branch type (prefix) in rimba rename

[lugassawan]

Summary

• Add RenameParams struct to internal/operations/rename.go (NewPrefix "" = inherit from branch)
• Extend rimba rename to accept an optional [new-task] arg and prefix flags (--bugfix, --hotfix, --docs, --test, --chore) so users can change the branch type without recreating the worktree
• Output now shows full branch names (feature/auth -> bugfix/auth) instead of task names
• No-op guard updated to "nothing to change: branch is already <branch>" (exit 1)

Test Plan

• Unit tests pass (make test)
• Linter passes (make lint)
• E2E tests pass (make test-e2e)

Type-tailored checks ([feat])

• rimba rename auth --bugfix → bugfix/auth branch and bugfix-auth dir created; feature/auth gone
• rimba rename auth login --bugfix → bugfix/login (rename + retype in one step)
• rimba rename auth (no flag, no change) → exit 1, stderr contains "nothing to change"
• Task-only rename still inherits prefix (existing TestRenamePreservesPrefix stays green)
• Coverage ≥ 97% (actual: 97.3%)

Issue

Closes #329

[lugassawan]

Medium (advisory): ResolveTaskInput is called on newTask here but the source task goes through findWorktree which internally re-resolves it. The code is functionally correct — monorepo inputs like auth-api/login work correctly because findWorktree handles the service prefix. The asymmetry is subtle though: a reader of this function must trace into findWorktree to understand why task doesn't also go through ResolveTaskInput at this call site. Consider explicitly resolving both here and passing the resolved task to findWorktree so the data flow is auditable in one place. Not a blocker, just a readability note.