docs: add direnv recipe for per-directory categories

[timvw]

Documents the per-directory category workaround that works today, as agreed in #138.

Why

docs/configuration.md documents {.env.WT_CATEGORY} as the way to group clones and worktrees, but the only ways to supply the value were per-command (WT_CATEGORY=work wt clone ...) or a single machine-wide export. Neither expresses "everything under this directory is work".

direnv covers that case with no wt changes at all — wt reads the environment. Since #139 landed {.env.X:-default}, a directory without an .envrc degrades to the default instead of hard-erroring, which is what makes the recipe worth recommending rather than merely possible.

What

• New ### Setting the category per directory subsection in docs/configuration.md, under Clone placement
• Short pointer to it from the README grouping example

The recipe places an .envrc under both repo_root and worktree_root for a category. That is deliberate: wt clone lands the checkout under repo_root/<cat>/..., and a later wt create from inside it needs the variable still set, or the worktree falls back to personal and the category is lost on the hop between the two trees.

Verification

• go build ./... && go test ./... — pass (docs-only change, run for the checklist)
• Confirmed by running it that direnv allow takes a single path, and that direnv exports propagate into nested subdirectories
• README anchor docs/configuration.md#setting-the-category-per-directory resolves to the new heading

Refs #138

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.14%. Comparing base (af50e8c) to head (664a90c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #143      +/-   ##
==========================================
+ Coverage   43.06%   43.14%   +0.08%     
==========================================
  Files          34       34              
  Lines        3597     3597              
==========================================
+ Hits         1549     1552       +3     
+ Misses       2048     2045       -3     

see 1 file with indirect coverage changes

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