fix(resume): resolve branch names Unicode-normalization-insensitively (NFC/NFD)

[KC1706]

Fixes #2551.

Problem

entire session resume <branch> compared and looked up branch names byte-for-byte, with no Unicode normalization. Git treats a precomposed (NFC, é U+00E9) and decomposed (NFD, e + U+0301) spelling of the same accented branch as one branch (core.precomposeUnicode), so git checkout finds it either way — but entire would report branch '<name>' not found locally or on origin when the supplied name's normalization differed from the on-disk ref. It bit cross-platform teams: a branch created on Linux (NFD) and synced couldn't be resumed on a macOS checkout (NFC), especially once refs were packed (git gc), where go-git matches packed-refs byte-exact.

Fix

• ResolveLocalBranchName (git_operations.go): exact ref match wins; otherwise it scans local branches and matches normalization-insensitively (fold to NFC), returning the canonical on-disk ref name so the checkout and the downstream checkpoint lookup target the real branch. Works on both NFC (macOS) and NFD (Linux) checkouts.
• branchNamesEquivalent: NFC-folded comparison with an ASCII fast path, used for the "already on this branch" check that previously failed even while standing on the branch.
• switchToBranchForResume now returns the resolved canonical name and threads it through resumeFromCurrentBranch (and the two trail resume callers), so the rest of resume uses the real ref name.

golang.org/x/text was already an (indirect) dependency; this promotes it to direct.

Testing

• New regression tests (git_operations_test.go): TestResolveLocalBranchName_UnicodeNormalizationInsensitive builds an NFD-stored branch with packed refs (deterministic across platforms via core.precomposeUnicode=false) and asserts the NFC spelling resolves to the canonical NFD ref; TestBranchNamesEquivalent_NFCvsNFD covers the comparison helper.
• mise run fmt && mise run lint → 0 issues. Existing resume/trail/branch tests pass.
• Manually reproduced the original failure and confirmed the fix against the built CLI (steps in entire session resume <branch> reports "branch not found" for a Unicode-equivalent (NFC vs NFD) branch name that git resolves fine #2551).

Scope

Deliberately scoped to the resume path from #2551. Other branch-name comparison sites (e.g. dispatch --all-branches) share the same underlying gap and could reuse these helpers in a follow-up; happy to do that separately if you'd prefer.

[KC1706]

Rebased onto latest main — the earlier conflicts (trail_cmd.go, go.mod) are resolved and it's now a single clean commit. Verified locally: build, the NFC/NFD regression tests, and mise run lint all pass. Ready whenever you have a moment.