feat(setup): weave setup --global (#110)

[rs545837]

Closes #110.

Adds a weave setup --global flag that configures git in ~/.gitconfig and writes the supported file-type rules to git's global attributes file (~/.config/git/attributes, or core.attributesfile if set), so weave becomes the default merge driver in every repo with no per-repo setup — the mergiraf-style flow @mikkelam asked for. No git repo required to run it. Documented in the README.

(#107 and #24 are covered by @edouard-andrei's #112 and #113.)

[inspect-review]

inspect review

Triage: 7 entities analyzed | 0 critical, 0 high, 4 medium, 3 low
Verdict: standard_review

Findings (4)

1. [low] Conflicting flags: local and global can both be true simultaneously, leading to undefined behavior. In attributes_target(), if both are true, global takes precedence and local is silently ignored. This violates user expectations and could cause confusion.
2. [low] Git repo check is bypassed when global=true, but git_path() is still called when local=true and global=false. If user runs weave setup --local outside a git repo, the code will fail at git_path("info/attributes") with a confusing error instead of the clear "Not in a git repository" message.
3. [low] In global_attributes_path(), the tilde expansion only handles ~/ prefix but git config can return other forms like ~username/path. This will fail to expand correctly and create a literal ~username directory.
4. [low] In global_attributes_path(), when core.attributesfile is not set or empty, the fallback path uses HOME or USERPROFILE environment variables. However, on Windows, the correct path should use USERPROFILE with a different structure (not ~/.config/git/attributes). The code assumes Unix-style paths for the default case, which will create incorrect paths on Windows.

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Reviewed by inspect | Entity-level triage found 0 high-risk changes