fix(powershell): source shellenv via Out-String so the profile line loads

[timvw]

Fixes #130.

The bug

The $PROFILE line wt init powershell writes (and the docs tell users to add
by hand) does not work:

Invoke-Expression (& wt shellenv powershell)

shellenv emits many lines, so the subexpression evaluates to Object[], and
Invoke-Expression's -Command parameter is [string]. The call is rejected,
the wt function is never defined, and auto-cd, tab completion and the
interactive menus are all silently absent — wt keeps working because wt.exe
is on PATH, so the failure is easy to miss once the error scrolls away.

I reproduced the exact error on PowerShell 7 as well, so this is not limited to
Windows PowerShell 5.1.

The fix

wt shellenv powershell | Out-String | Invoke-Expression

Hoisted into a single powershellSetupLine constant so wt init, the
shellenv help text and docs/installation.md cannot drift apart again.

Checked the obvious objection to Out-String: it does not wrap the 220-char
completer line even at host width 80, so nothing is truncated.

Why CI missed it

e2e/run.go sourced shellenv through a harness-only variant that pre-joined the
lines, so the PowerShell legs exercised an invocation no user is ever told to
write while the shipped one stayed broken and green.

• The harness now uses the documented pipeline verbatim (modulo $env:WT_BIN).
• It also passes the shell explicitly, matching what the bash leg already did —
  previously a stray MSYSTEM/SHELL could have made it auto-detect bash and
  feed bash code into Invoke-Expression.
• TestPowerShellSetupLineHasNoDrift pins docs/installation.md and
  e2e/run.go to the constant and fails if either regresses to the
  subexpression form. I verified the guard actually fires by reintroducing the
  old line in a scratch copy.

Verification

Dispatched CI on this branch before opening the PR: run 32257055607 — all green.

The E2E (windows, powershell) leg is Windows PowerShell 5.1, the environment
from the issue: 77 scenarios passed, 0 failed. Those scenarios assert
cwd_ends_with, which only passes if the wt function performed
Set-Location — wt.exe alone cannot change its parent shell's directory. So
this is real evidence the integration loaded, not the false pass the issue
warned about.

Known gap, not addressed here

The five PowerShell PTY tests in cmd/e2e_interactive_test.go are aligned to
the documented form too, but they never run in CI: they are guarded by
testing.Short() and both unit-test jobs run go test -short. Worth a separate
issue; out of scope for this fix. The real coverage comes from e2e/run.go.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 37.82%. Comparing base (8161a2f) to head (d139e17).

Files with missing lines	Patch %	Lines
e2e/run.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #135      +/-   ##
==========================================
+ Coverage   37.76%   37.82%   +0.06%     
==========================================
  Files          33       33              
  Lines        3509     3506       -3     
==========================================
+ Hits         1325     1326       +1     
+ Misses       2090     2085       -5     
- Partials       94       95       +1     

Files with missing lines	Coverage Δ
cmd/init.go	49.09% <100.00%> (-0.46%)	⬇️
cmd/shellenv.go	80.05% <ø> (ø)
e2e/run.go	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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