fix: implement install-dir override chain in the installer

[SomSamantray]

Issue for this PR

Closes #43772

Related: #42974 (duplicate report of the same root cause)

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

README.md documents a four-level priority chain for the install directory ($OPENCODE_INSTALL_DIR -> $XDG_BIN_DIR -> $HOME/bin -> $HOME/.opencode/bin), but the install script ignored both env vars and always installed to $HOME/.opencode/bin. This implements the chain exactly as documented: check $OPENCODE_INSTALL_DIR, then $XDG_BIN_DIR, then try creating $HOME/bin, and only fall back to $HOME/.opencode/bin when none of those apply. No other install-script behavior changes.

How did you verify your code works?

No test harness exists for the install shell script in this repo, so I exercised the modified block in isolation with env -i to control HOME/env vars per case:

• $OPENCODE_INSTALL_DIR set alone -> used
• $XDG_BIN_DIR set alone -> used
• both set -> $OPENCODE_INSTALL_DIR wins
• neither set, $HOME/bin missing but creatable -> created and used
• neither set, $HOME/bin already exists -> used
• neither set, $HOME/bin uncreatable (read-only $HOME) -> falls back to $HOME/.opencode/bin
• $OPENCODE_INSTALL_DIR exported empty -> treated as unset, falls through

All 8 checks passed. Also ran bash -n install (syntax check) and confirmed git diff touches only the INSTALL_DIR resolution block (lines 68-69 originally).

Screenshots / recordings

N/A — shell script change, no UI.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found:

• PR fix(install): honor OPENCODE_INSTALL_DIR and XDG_BIN_DIR as documented #42975: fix(install): honor OPENCODE_INSTALL_DIR and XDG_BIN_DIR as documented
  • fix(install): honor OPENCODE_INSTALL_DIR and XDG_BIN_DIR as documented #42975

This PR appears to address the same issue—implementing the install directory override chain for OPENCODE_INSTALL_DIR and XDG_BIN_DIR. Since the current PR is #44079 and #42975 already exists with nearly identical scope, you should check whether #42975 is still open or if there's been progress that makes the current PR redundant.