fix(installer): skip pull after checking out a tag

[Spelt]

Problem

Installing a dependency pinned to a version tag (e.g. boss install https://host/owner/repo:1.2.3) surfaces a warning on every install/update:

⚠️ Installation Warnings:
   - <dep>: Error on pull from dependency <repo>
command failed: exit status 1
Stderr: You are not currently on a branch.
Please specify which branch you want to merge with.

Checking out a tag leaves the module worktree in detached HEAD state, and checkoutAndUpdate unconditionally runs git pull afterwards — which git refuses without a current branch.

Fix

A tag is a fixed reference; there is nothing to pull into it. Only pull when the checked-out reference is a branch (referenceName.IsBranch()). The line-ending normalization step still runs for both cases.

Testing

• go build ./..., go vet clean; go test ./internal/core/services/installer/... passes
• Windows 11: boss install https://github.com/viniciussanchez/dataset-serialize:2.5.9 previously warned "You are not currently on a branch" on the pull; with this patch the install completes with no warnings, and branch-based dependencies still pull updates as before

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@1ea375d). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
internal/core/services/installer/core.go	0.00%	8 Missing ⚠️
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #284   +/-   ##
=======================================
  Coverage        ?   28.94%           
=======================================
  Files           ?       90           
  Lines           ?     5701           
  Branches        ?        0           
=======================================
  Hits            ?     1650           
  Misses          ?     3908           
  Partials        ?      143           

Flag	Coverage Δ
unittests	28.94% <0.00%> (?)

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