fix: restore macOS Bash 3.2 compatibility

[EdgeClaws]

Summary

• restore compatibility with macOS system Bash 3.2
• keep Python heredocs out of command and process substitutions
• preserve the existing target-resolution and Gateway session-patch behavior

Root cause

Bash 3.2 cannot reliably parse heredocs nested directly inside $(...) or
< <(...). The current script exits during parsing before it can touch any
files.

The fix moves readable heredoc bodies into helper functions and uses
python3 -c for the two small command-substitution cases.

Verification

• /bin/bash -n scripts/switch.sh — passes on macOS Bash 3.2.57
• /opt/homebrew/bin/bash -n scripts/switch.sh — passes on Bash 5.3
• Gateway override fixture with two sessions — attempted=2 ok=2 failed=0 under both Bash versions
• provider/model dry run — resolves and completes under Bash 3.2
• git diff --check — passes

PR #4 also modifies scripts/switch.sh, so whichever PR lands second may need
a straightforward rebase.