Optimize install.sh to avoid unnecessary rebuilds and reuse unchanged build artifacts

[OpenCodeEngineer]

Problem

./install.sh already skips installation-time rebuilds in some cases, but it still leaves obvious build-speed wins on the table.

Current behavior in install.sh:

• it uses a coarse stale check based on git describe and file mtimes under codex-rs/
• it forces CARGO_INCREMENTAL=0, which prevents reuse of incremental artifacts during local rebuilds
• it only decides whether to rebuild the final binary, not whether unchanged intermediate artifacts can be reused more aggressively

The result is slower local install/build loops than necessary, especially when only a small subset of files changed.

Goal

Make ./install.sh faster for iterative local use by avoiding unnecessary rebuild work and reusing build outputs when inputs have not changed.

Acceptance Criteria

• Audit the current rebuild decision logic in install.sh and identify avoidable full rebuild paths.
• Propose or implement a caching/reuse strategy for unchanged inputs and build artifacts.
• Preserve correctness: modified sources must still trigger rebuilds, and stale binaries must not be installed.
• Document any new env knobs or behavior changes in ./install.sh --help.
• Add validation that demonstrates the optimized path skips redundant rebuild work when inputs are unchanged.

Notes

Likely areas to investigate:

• whether CARGO_INCREMENTAL=0 is still the right default for local install flows
• whether the stale check should use a more precise fingerprint than broad file mtime scanning
• whether build metadata can be cached to distinguish source changes from no-op installs
• whether release/profile tradeoffs should differ for first build vs repeat local installs

[OpenCodeEngineer]

Created from follow-up request to speed up ./install.sh by avoiding unnecessary rebuilds and reusing unchanged build artifacts. Status: tracked, not yet implemented in this worktree.

[OpenCodeEngineer]

Implemented and pushed on branch issue-22-install-cache. PR is up: #24.

What changed:

• added a repo-tracked source-building ./install.sh;
• switched the stale check from pure mtimes to a build stamp + content fingerprint so unchanged source content does not trigger rebuilds;
• enabled incremental release builds by default in fast mode;
• honored CARGO_TARGET_DIR so build caches can be reused across installs;
• added a shell test that verifies unchanged content skips rebuilds, content changes rebuild, and custom target dirs are reused.

Validation run:

• sh scripts/install/test-source-build-install.sh
• sh -n install.sh scripts/install/test-source-build-install.sh
• sh install.sh --help