Add ~/.local/bin/git-ai symlink in install.sh

[svarlamov]

Add ~/.local/bin/git-ai symlink in install.sh

Summary

Adds a ~/.local/bin/git-ai symlink during installation so that git-ai is immediately available on systems where ~/.local/bin is already on PATH (common default on Ubuntu, Fedora, and other Linux distros).

• install.sh (Linux/macOS): Creates a symlink at ~/.local/bin/git-ai → ~/.git-ai/bin/git-ai. Wrapped in non-fatal if/else with 2>/dev/null so failures don't abort the install under set -euo pipefail.
• CI: Adds verification for the symlink in the install-scripts-local workflow (Unix only).
• Windows: No changes — ~/.local/bin is not a standard PATH directory on Windows, and the existing install.ps1 already handles User PATH updates for ~/.git-ai/bin without admin.

Closes #597

Review & Testing Checklist for Human

• Unconditional ~/.local/bin creation: The installer creates ~/.local/bin/ via mkdir -p even if it didn't previously exist. Verify this is acceptable — some users may not want this directory created (especially on macOS where ~/.local/bin is less standard).
• Manual install test: Run install.sh locally and verify ~/.local/bin/git-ai exists as a working symlink pointing to ~/.git-ai/bin/git-ai.

Notes

• Requested by: @svarlamov
• Link to Devin run

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