[AUTOMATION] fix: optimize build env assembly

[michiosw]

Summary
This optimizes build env assembly by preallocating the final slice once.

Before this, resolved credential entries were appended onto a copied base slice in internal/credential/credential.go, which left the runtime env path doing extra growth work on every run.

Now one pre-sized slice is the canonical path:

env := make([]string, 0, len(base)+len(resolved))
env = append(env, base...)

Why
This gives kontext-cli a cheaper runtime path for env setup:

run startup
-> credential.BuildEnv
-> final agent env slice

This PR does not broaden behavior beyond the optimization scope.

What changed
Optimized env slice assembly in internal/credential/credential.go
Removed repeated slice growth while appending resolved credentials
Preserved env ordering and base-slice isolation
Updated tests for base-slice mutation and output order

Verification
go test ./internal/credential
go test ./internal/run
go test ./...
go vet ./...
git diff --check

[michiosw]

• [AUTOMATION] fix: optimize build env assembly #289 👈 (View in Graphite)
• main

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[greptile-apps]

Greptile Summary

This PR optimizes credential environment assembly without changing the intended output order. The main changes are:

• Preallocates the final environment slice in credential.BuildEnv.
• Copies base environment entries with append(env, base...) before resolved credentials.
• Adds coverage for output order and base-slice isolation.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

_{Reviews (1): Last reviewed commit: "fix: optimize build env assembly" | Re-trigger Greptile}