feat(io): centralize shared IO utilities

[aidandaly24]

Description

This PR introduces a shared src/io package for CLI input and output concerns:

• Moves AppIO, ReadWriteJson, and FsReadWriteJson into src/io and migrates existing consumers.
• Adds a class-based SourceResolver for inline values, file:// sources, and stdin, with byte and strict UTF-8 text resolution.
• Enforces a single stdin consumer and provides consistent SourceResolutionError failures.
• Supports optional AbortSignal cancellation for file and stdin reads, following the cancellation behavior introduced in feat(runtime): add Runtime invoke command #1820.

The io package has no dependency on handlers or global config. Existing exports were removed rather than temporarily re-exported so dependent PRs can rebase onto the intended package boundary.

Related Issue

N/A - this is maintainer-coordinated refactor work and does not have an associated issue.

Documentation PR

N/A

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe): Internal package refactor

Testing

How have you tested the change?

• I ran bun test (345 passed, 0 failed)
• I ran bun run typecheck
• I ran bun run lint:check
• I ran bun run format:check
• I ran bun run build
• src/assets/ was not modified, so snapshots do not require updates

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published, or no published dependency is required

---

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terms of your choice.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.89%. Comparing base (e0de6ed) to head (255362b).

Additional details and impacted files

@@             Coverage Diff              @@
##           refactor    #1832      +/-   ##
============================================
+ Coverage     94.85%   94.89%   +0.04%     
============================================
  Files           149      151       +2     
  Lines          7360     7421      +61     
============================================
+ Hits           6981     7042      +61     
  Misses          379      379              

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[jariy17]

Overall, great CR! Left a few comments. Can we also update the README.md about this new SourceResolver so our agents try to use this?

[jariy17]

Did we discuss about making all Errors, "AgentCoreErrors"? If so, we need to change this after @Hweinstock merges the AgentCore PR.

[jariy17]

Can we update this test to abort after the file read starts? Aborting before resolveBytes() means throwIfAborted() exits early, so this never verifies that the signal reaches readFile().