ext: derive type: path mounts from config, drop ext-paths.json

[mobileoverlord]

Summary

Makes avocado.yaml the single source of truth for type: path extension
mounts. Previously avocado ext fetch recorded resolved mounts in
.avocado/ext-paths.json, which was read both at container launch (for bindfs
mounts) and during config composition. That side-table could drift from the
config whenever an extension's source flipped between package and path —
a stale entry kept mounting a host path over an extension that was now a
package.

Details

• SdkContainer::derive_ext_path_mounts reads + interpolates avocado.yaml and
  mounts every source: { type: path } extension (replaces the state-file read
  at container launch).
• Config composition resolves a path-based extension's config directly from its
  declared source.path instead of the state file.
• Removes ExtensionPathState entirely; ext fetch for a path extension is now
  validation-only (nothing is written).

Flipping an extension between package and path now takes effect immediately
with nothing to reconcile.

Follow-up (out of scope)

A separate change will clear a previously package-populated includes/<ext>
when switching it to path, so bindfs mounts onto a clean directory.

Testing

• Added tests for deriving mounts from config (a path source is mounted, a
  package source is ignored; no path sources → no mounts).
• cargo build, cargo test, and cargo clippy are green.

[Copilot]

Pull request overview

This PR makes avocado.yaml the single source of truth for type: path extension mounts by removing the persisted .avocado/ext-paths.json state and instead deriving path mounts directly from the config at container launch, while also resolving path-based extension configs directly from their declared source.path.

Changes:

• Removed ExtensionPathState and stopped writing .avocado/ext-paths.json; ext fetch for type: path is now validation-only.
• Added SdkContainer::derive_ext_path_mounts to parse + interpolate avocado.yaml and derive bindfs mounts for source: { type: path } extensions.
• Updated config composition to read a path extension’s config directly from its declared source.path instead of consulting a state file; added tests for mount derivation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/utils/ext_fetch.rs	Removes persisted path-mount state and makes path-extension fetch validation-only.
src/utils/container.rs	Derives type: path bindfs mounts from avocado.yaml at container launch; adds tests.
src/utils/config.rs	Resolves path-based extension configs directly from source.path during composition.

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