fix(shell): use bash -l only for buildpack apps

[ipmb]

Docker/Dockerfile-based apps may set PATH in the image. Launching the shell with bash -l sources /etc/profile, which can clobber that PATH. Reserve bash -l for buildpack apps (where the CNB launcher wants login-shell semantics) and use plain bash otherwise.

Refactor shell task lookup along the way: memoize the DescribeTaskDefinition call with sync.Once, and split ShellTaskFamily into focused ShellTaskFamily and IsBuildpack accessors so callers no longer discard half the return.

[Copilot]

Pull request overview

Adjusts interactive shell startup to avoid bash -l clobbering image-provided PATH for Dockerfile/Docker-based apps, while keeping login-shell semantics for buildpack-based apps. It also refactors shell task metadata lookup to avoid repeated ECS DescribeTaskDefinition calls.

Changes:

• Use bash -l only for buildpack apps; use plain bash otherwise.
• Memoize shell task definition lookup with sync.Once and cache task family/tags.
• Split the previous combined API into ShellTaskFamily() and IsBuildpack() accessors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cmd/shell.go	Chooses bash vs bash -l based on build system and uses IsBuildpack() for buildpack-specific exec wrapping.
app/app.go	Caches shell task definition metadata via sync.Once; adds IsBuildpack() and simplifies ShellTaskFamily() return shape.

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