feat: auto-load .env.<stackname> for per-stack env overrides

[simple-agent-manager]

Closes #2128

Summary

• add LoaderOptions.EnvFiles and wire additional env files into compose-go while preserving default .env loading
• discover .env. after stack resolution and pass it to the compose loader when present
• add loader/session coverage plus package testdata fixtures for precedence, missing/empty files, unresolved placeholders, and stack-only env files

Validation

• go test -short ./...
• make lint GO-NO-DEP-GO=1

Note: plain make lint currently fails during local Go bootstrap because the Makefile constructs an invalid go1.25.5.0 download URL from go.mod; GO-NO-DEP-GO=1 uses the installed go1.25.10 toolchain and passes.

[simple-agent-manager]

Code Review Summary (3-round CTO adversarial review)

I ran this PR through 3 rounds of back-and-forth with a skeptical CTO subagent. Here's what was found and addressed:

Issues Found & Fixed (pushed as e190205f)

1. Redundant Loader instantiation — stackEnvFiles() was creating an entire compose.Loader (with full env loading, config path resolution, etc.) just to determine the working directory. Replaced with simple filepath.Dir(configPaths[0]) / os.Getwd() fallback.

2. Double-append risk — withEnvFiles() is called twice in newProjectOptions (before and after config path discovery). Without deduplication, stack env files could be appended twice. Added slices.Contains guard.

3. Silent behavior on load — When a .env.<stack> file IS picked up, the user had no feedback. Added term.Infof("Loading stack environment from %s") so users can trace which files affect their deployment.

4. Missing documentation — The directory check (filepath.Dir(absEnvFile) == wd) was undocumented, making it look like dead code. Added comments explaining why it exists (prevents cwd-resolved paths leaking into COMPOSE_FILE-discovered projects).

5. Precedence model undocumented — Added doc comment explaining that compose-go's dotenv parser uses last-wins semantics (verified by reading source at compose-go/v2/dotenv/env.go:GetEnvFromFile).

What Looked Good (no changes needed)

• Path traversal safety: ValidateStackName enforces ^[a-z][a-z0-9]*$, so stack names can't contain special chars
• Defensive copy of EnvFiles slice in session.go (prevents mutation of shared options)
• Test coverage: 4 compose-level tests + 3 session-level test cases covering precedence, missing files, empty files
• The CreateProjectForDebug refactoring (using GetWorkingDir() instead of os.Getwd()) is a net improvement for that method's correctness

Remaining Notes (not blockers)

• Empty ConfigPaths case: when compose file is discovered via COMPOSE_FILE env or directory walking, stack env resolution falls back to os.Getwd(). This is correct for the standard workflow (user runs from project dir).
• Feature is intentionally NOT opt-in — .env.<stack> follows established conventions (.env.local, .env.production). Zero impact on users who don't create the file.

[lionello]

Feels very, very sloppy. Agree with intent.

[lionello]

rename folder to envstackfixture to avoid another level of indirection.