security: fix path traversal in agent builder and documentation micro-fixes

[Kaushikj-7]

Description

This PR addresses a critical security vulnerability and improves documentation clarity.

Type of Change

• Bug fix (non-breaking change that fixes a security issue)
• Documentation update
• New feature (added path validation unit tests)

Related Issues

N/A (Security audit finding)

Changes Made

• Security: Added robust path validation to agent_builder_server.py to prevent path traversal in
  session and export tools.
• Testing: Added a new test suite core/tests/test_agent_builder_path_traversal.py using unittest
  to verify protection against malicious paths.
• Docs: Fixed typos and clarified terminology in getting-started.md, developer-guide.md, and other
  core files.

Testing

• Unit tests pass (python -m unittest core/tests/test_agent_builder_path_traversal.py)
• Manual verification of security fix using a reproduction script.

Checklist

• My code follows the project's style guidelines
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally

[Kaushikj-7]

1. Vulnerability Addressed:
   I identified a path traversal risk in agent_builder_server.py where user-provided session_id and
   agent_path parameters were used directly in filesystem operations (unlink, mkdir, read_text). A
   malicious input could have allowed an attacker to read or delete files outside of the intended
   directories.

2. The Fix:

• _safe_path_segment: Now strictly enforces that session IDs contain no path separators (/, )
  or parent directory references (..).
• _safe_export_path: Uses .resolve() and a prefix check to ensure that all agent exports and
  imports are strictly confined to the exports/ directory, preventing escapes to sensitive system
  areas.

3. Test Coverage (see core/tests/test_agent_builder_path_traversal.py):
   I added a new test suite that verified the following:

• Success Cases: Valid alphanumeric session IDs and local export paths work as expected.
• Traversal Blocking: Confirmed that inputs like ../../etc/passwd or sub/dir trigger a ValueError.
• Integrity Check: Verified that delete_session successfully blocks a deletion attempt targeting a
  file outside the restricted sessions directory.

4. Documentation:
   Corrected terminology for "builder skills" to match the project's internal naming conventions and
   fixed minor typos.