Fix the project-directory advice #2283 added (core fixes)

[schnie]

Replaces #2288 with the core user-facing fixes to the project-directory advice #2283 added, built on v2's existing helpers (config.IsProjectDir, project.HasManifest). The only change under config/ is that IsProjectDir's existing home check now compares paths properly (see 6). Non-test lines vs v2: +102 / -15.

Fixes

1. APC astro deploy --dags no longer needs a Dockerfile. A DAG-only deploy builds nothing; it uploads dags/. cmd/apc's EnsureProjectDir var (name kept) now points at ensureDeployProjectDir. For --dags it accepts a pyproject.toml project (project.HasManifest) or a 1.x project (utils.EnsureProjectDir, with no Dockerfile check). Every other APC deploy still goes through utils.EnsureDockerfileProjectDir.
2. APC astro deploy --remote skips the project check. --remote points the Deployment at an image already in the registry and builds nothing, so a missing --image-name is reported by RunE ("--image-name is mandatory when --remote flag is passed") rather than as a missing Dockerfile. --remote --dags still checks the project, since RunE makes a DAG-only deploy then.
3. --image-name is read by its value in PreRunE and RunE alike. In astro deploy (Astro) and APC deploy, an explicit empty --image-name= names no image. The project check runs, and on Astro, --image-name= --dags is no longer refused as "--image-name implies an image-only deploy".
4. APC astro deploy --save refuses a pyproject.toml project that has no .astro/config.yaml, with a usage error: "--save stores the deployment in .astro/config.yaml, which a pyproject.toml project does not have; pass the deployment id on each deploy instead". Without this, saving would create .astro/config.yaml and turn the project into a 1.x one.
5. EnsureDockerfileProjectDir also requires <WorkingPath>/Dockerfile, which the APC build parses there. The error names what is missing ("this directory has no .astro/config.yaml and no Dockerfile" / "has no Dockerfile"), and its advice can be followed: run astro package --tag <image>, then astro deploy <deployment-id> --image-name <image>.
6. Home directory.
   • config.IsProjectDir's home check used a raw HomePath == path. It now calls config.IsHomeDir, which compares the cleaned paths and falls back to os.SameFile, so ~/ and a symlink to ~ are not project directories either.
   • When the working directory is home, both checks in cmd/utils use that same helper to say so, and they do not suggest astro init.

astro remote deploy is unchanged from v2: the pyproject.toml bypass stays, and --image-name does not skip the project check. DeployClientImage reads the registry it pushes to from the project's .astro/config.yaml, so outside a project it could push to the wrong registry. TestRemoteDeployImageNameStillChecksTheProject pins this.

Tests:

• TestDeployProjectCheckFollowsTheDeploy (cmd/apc): covers --dags, --remote and --save. It restores every global it sets.
• TestDeploySkipsEnsureProjectDirWhenImageNameSet and TestDeployEmptyImageNameIsNotImageOnly (cmd/astro)
• TestRemoteDeployImageNameStillChecksTheProject
• TestEnsureDockerfileProjectDirNamesWhatIsMissing
• TestEnsureProjectDirInTheHomeDirectory
• TestIsProjectDirRefusesHomeUnderAnySpelling (config)

Left out on purpose (follow-ups)

#2288 rewrote project detection to cover these, and each review round found new regressions in that machinery (#2288's reviews). Each needs its own design:

• An astro init guard against scaffolding into $HOME. An earlier revision of this PR refused it, but that breaks containers and CI where HOME is the project root (Astro Runtime images use HOME=/usr/local/airflow). The advice no longer sends anyone to run init in ~.
• Enclosing-project advice from subdirectories, i.e. "you are inside the project at X". This needs a directory walk with home and settings-file stops.
• A unified "is a project" predicate across 1.x .astro/config.yaml and pyproject.toml manifests.
• ASTRO_HOME settings-file handling: the CLI's own ~/.astro/config.yaml, or one under ASTRO_HOME, can be mistaken for a 1.x project's.
• astro deploy --save on Astro from a pyproject project, which may need the same refusal APC deploy now has.
• astro init's warning when nesting a project inside another.

Checks

• go build ./... and GOOS=windows go build ./...
• GOOS=windows go vet on config, cmd/utils, cmd/apc, cmd/astro and cmd/local
• make test and make test-e2e (tier 0)
• make lint (no autofix rewrites), make lint-goos and make deadcode

Merge-tree

git merge-tree --write-tree is conflict-free against all three open branches. Each combination also passes go vet ./cmd/... ./config/ and go test on the touched packages:

• origin/apc-deploy-warn-no-dags (Warn when an APC --image-name deploy ships only the image's DAGs #2284)
• origin/init-keep-apc-dockerfile (Refuse to convert a 1.x project under an APC context, and give the same advice everywhere #2285)
• origin/tombstone-v1-flags (Tell a 1.x script what replaced each flag v2 dropped silently #2280), where go test covered all of ./cmd/...

[coveralls-official]

Coverage Report for CI Build 37941716828

Coverage increased (+0.03%) to 57.496%

Details

• Coverage increased (+0.03%) from the base build.
• Patch coverage: 6 uncovered changes across 2 files (57 of 63 lines covered, 90.48%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
cmd/utils/utils.go	23	19	82.61%
cmd/apc/deploy.go	25	23	92.0%
Total (4 files)	63	57	90.48%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	69095
Covered Lines:	39727
Line Coverage:	57.5%
Coverage Strength:	199.61 hits per line

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💛 - Coveralls