feat(pattern): allow default values for env vars via {.env.X:-fallback}

[timvw]

Summary

• Pattern variables referencing environment variables now support bash-style defaults: {.env.WT_CATEGORY:-personal} uses personal when WT_CATEGORY is unset
• Default values respect the configured separator (e.g. with separator = "-", {.env.X:-a/b} yields a-b)
• Hard-failing when a variable has neither a value nor a default is preserved as a design property, not an incidental leftover — it is what makes a misspelled {.brnach} a loud failure rather than a silently mangled path

Closes #136

What this changes

The hard error on a missing variable is intended behaviour and stays. What was missing is a way to opt out of it for a specific variable. The three cases are now:

Situation	Behaviour
value available	use it
no value, default provided	use the default
no value and no default	hard error — unchanged

Previously only the first and third existed. PR #127 documents {.env.WT_CATEGORY} as the recommended way to group clones by category, so a user who has not exported the variable gets the error correctly but has no way to declare a fallback in the pattern.

Syntax: {.env.X:-fallback} (bash parameter expansion style)

The implementation preprocesses the pattern string to rewrite {.env.X:-fallback} into an internal envOr template function call before template.Parse, keeping missingkey=error in force for every other reference.

Alternatives rejected

• {envOr "X" "default"} (template function) — trivially correct but drops the bash-like ergonomics that were asked for.
• {.env.X | default "default"} (sprig pipeline) — cannot work with missingkey=error, because the map lookup errors before the pipeline runs. Adopting it would mean relaxing the very option that produces the wanted error.

What still errors

• {.env.UNSET_VAR} (no :-) — hard error, same as before
• {.brnach} or any misspelled non-env key — hard error, same as before

These are the headline guarantees of this change, and both are pinned by tests.

Scope

Adds the default-value syntax for {.env.*} references in both pattern (worktree placement) and repo_pattern (clone placement). It does not add {.var.X}, a [vars] config section, or wt.var.* git config keys — that is #137. Nor does it address where a per-group value comes from — that is #138. Tracking issue: #132.

Test coverage

• Unit tests (internal/tmpl/tmpl_test.go): default used when unset, default ignored when set, empty default, slash in default, separator applied to default, set-to-empty-string uses value not default, multiple defaults, mixed with plain env ref, unset without default still errors, misspelled non-env key still errors
• E2E scenarios (e2e/scenarios/env-vars.yaml): default used when unset, default ignored when set, empty default, slash in default, separator applied to default
• Red-green verified: broke envOr to always return the fallback, confirmed the "default ignored when set" test failed, restored

Test plan

• go test ./... — 531 passed
• go vet ./... — clean
• gofmt -l . — clean
• go run e2e/run.go — 247 passed, 0 failed
• Codex review loop (2 rounds): separator inconsistency fixed, wt info output updated

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.56%. Comparing base (8d63b05) to head (d46a5b4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/info.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #139      +/-   ##
==========================================
+ Coverage   41.50%   42.56%   +1.06%     
==========================================
  Files          34       34              
  Lines        3542     3580      +38     
==========================================
+ Hits         1470     1524      +54     
+ Misses       2072     2056      -16     

Files with missing lines	Coverage Δ
cmd/clone_path.go	69.23% <100.00%> (ø)
cmd/worktree_path.go	61.06% <100.00%> (ø)
internal/tmpl/tmpl.go	89.47% <100.00%> (+89.47%)	⬆️
cmd/info.go	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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