fix(hooks): replace BASH_SOURCE with POSIX-safe $0 for Linux compatibility

[jd316]

This PR completes the Linux/Ubuntu compatibility fix started in v4.3.1.

While the shell wrapper was updated, the internal session-start hook script still used the ${BASH_SOURCE[0]} syntax, which causes a 'Bad substitution' error on systems where /bin/sh is dash (like Ubuntu).

Changes:

• Fixed hooks/session-start to use $0.
• Updated docs/windows/polyglot-hooks.md to show the correct POSIX-safe pattern for future developers.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Review profile: CHILL

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📥 Commits

Reviewing files that changed from the base of the PR and between e4a2375 and a62f052.

📒 Files selected for processing (2)

• docs/windows/polyglot-hooks.md
• hooks/session-start

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📝 Walkthrough

Walkthrough

Updated path-resolution logic in hook scripts from ${BASH_SOURCE[0]} to $0 across documentation and execution scripts, along with documentation formatting enhancements for Windows polyglot hooks configuration.

Changes

Cohort / File(s)	Summary
Documentation docs/windows/polyglot-hooks.md	Added Windows-specific bullet point, formatting polish, and updated path-detection logic example in run-hook.cmd wrapper from ${BASH_SOURCE[0]} to $0 for SCRIPT_DIR computation.
Hook Scripts hooks/session-start	Changed SCRIPT_DIR resolution from ${BASH_SOURCE[0]:-$0} to $0.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• fix: restore polyglot wrapper for Windows hook compatibility (4.3.1) #523: Directly related; modifies identical path-resolution logic (${BASH_SOURCE[0]} to $0) in hook wrappers (run-hook.cmd and session-start).
• Fix session-start hook execution in plugin context #9: Related; also modifies SCRIPT_DIR path-resolution in hooks/session-start with similar bash variable pattern changes.
• Add Windows support for plugin hooks #134: Related; introduces run-hook.cmd polyglot wrapper and adjusts script-invocation path logic for Windows-compatible hook execution.

Poem

🐰 Hops through scripts with glee,
$0 now resolves the way to be,
From BASH_SOURCE's nested dance,
To simpler paths—cross-platform's chance! 🎯

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main functional change: replacing BASH_SOURCE with POSIX-safe $0 for Linux compatibility, which matches the primary changes in hooks/session-start and the updated documentation.
Description check	✅ Passed	The description is directly related to the changeset, explaining the rationale (Linux/Ubuntu compatibility fix), the specific problem ('Bad substitution' error with ${BASH_SOURCE[0]}), and listing the changes made (hooks/session-start and documentation updates).
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[jd316]

@obra
Hey! Please review and merge this PR.