chore: finish PowerShell script audit for issue #25

[EnmaJim]

Linked issue

Closes #25

Summary of changes

Completes the three remaining acceptance criteria on the script audit:
error handling, UTF-8 file encoding, and logging-helper consistency.

• Error handling standardised. Every .ps1 entry-point now sets
  Set-StrictMode -Version 3.0 and $ErrorActionPreference = 'Stop',
  variables are initialised before try blocks so strict-mode reads
  on early throw no longer surface as "property cannot be found",
  optional JSON-RPC and workflow-manifest fields are guarded under
  strict mode, and empty/silent-discard catch blocks have been
  replaced with Write-BotLog calls (atlassian-download, InboxWatcher
  worker log, ui/server port probe, etc.).
• UTF-8 handling verified. Every Set-Content / Add-Content /
  Out-File call now declares -Encoding utf8NoBOM, and
  .gitattributes pins LF line endings for *.ps1, *.psm1,
  *.psd1.
• Logging helpers consistent. Write-Warning and bare error
  prints in long-running paths are routed through Write-BotLog, and
  the strict-mode leak that broke /api/state (dot-sourcing a script
  with top-level Set-StrictMode from Get-BotState) is fixed at
  source: dot-sourceable scripts keep strict mode inside function
  bodies.
• Regression guards. Three new Layer-1 test files lock the rules
  in place so the audit cannot silently regress:
  • tests/Test-ErrorHandling.ps1 - entry-point directives + no
    empty / silent-discard catch blocks.
  • tests/Test-FileEncoding.ps1 - no BOM, explicit -Encoding,
    .gitattributes LF declarations.
  • tests/Test-DotSourceIsolation.ps1 - dot-sourced .ps1 files
    must not leak strict mode into the caller (runtime probe + AST
    lint).
• Workflow + Claude CLI robustness. Surfacing exit code/stderr on
  stdin write failure, regenerating the Claude session ID per retry
  attempt, cleaning session artefacts between retries, and a
  background stderr drain so worktree git chatter no longer leaks
  into pipelines.

Screenshots / recordings

Not applicable - no UI changes.

Testing notes

• pwsh tests/Test-ErrorHandling.ps1 - all rules pass.
• pwsh tests/Test-FileEncoding.ps1 - all rules pass.
• pwsh tests/Run-Tests.ps1 - full Layer-1 + component + workflow
  suites green, including the new Test-WorkflowSessionRetry,
  Test-RuntimeHelpers, Test-RouteHandlerSmoke, and
  Test-McpHelpers.

Checklist

• Tests added or updated
• Docs updated (if behaviour changed) - none required, behaviour
  unchanged
• Linked issue exists
• Follows the contribution guide