refactor: remove term-challenge code from platform-v2 (#53)

Migrate all term-challenge-specific code out of platform-v2 so it
remains a generic framework. Term-challenge now lives in its own
repository and imports platform-challenge-sdk-wasm as a git dependency.

Removed challenge crates:
- Delete challenges/term-challenge/ directory entirely
- Delete challenges/term-challenge-wasm/ directory entirely
- Remove both from workspace members in Cargo.toml
- Update Cargo.lock to drop term-challenge packages

Removed term-specific types from SDK:
- Delete crates/challenge-sdk-wasm/src/term_types.rs (CommandRequest,
  CommandResult, FileReadRequest/Response, FileWriteRequest/Response,
  FileListRequest/Response, FileEntry, TermEvaluationMetrics, etc.)
- Remove pub mod term_types and pub use term_types::* from SDK lib.rs
- Remove TermEvaluationParams struct from SDK types.rs
- Remove TermEvaluationParams from the re-export list in lib.rs

Generalized term-challenge references across platform-v2:
- wasm-runtime-interface: Rename SandboxPolicy::term_challenge() to
  default_challenge(), TerminalPolicy::term_challenge() to
  default_challenge(), update sandbox test accordingly, rename
  RuntimeConfig::term_challenge() to default_challenge()
- bins/platform-cli: Remove hardcoded term-challenge default config,
  use empty HashMap for challenges
- challenge-registry: Generalize comment from term-challenge to generic
- secure-container-runtime: Replace term-challenge test strings with
  generic test-challenge names in policy.rs, ws_transport.rs, and
  integration_tests.rs
- challenge-orchestrator (non-workspace): Replace term-challenge Docker
  volume names, container prefixes, and image references with generic
  names in docker.rs, evaluator.rs, and lib.rs

Updated documentation:
- AGENTS.md: Remove term-challenge from architecture table, mermaid
  diagram, and workspace crates list. Generalize code examples to use
  MyChallenge instead of TermChallenge.
- challenges/README.md: Remove term-challenge-specific build section,
  keep generic challenge instructions
- docs/validator_wasm_audit.md: Generalize term-challenge references

Updated build/deploy configs:
- docker/Dockerfile.challenge: Remove term-challenge-wasm build target,
  keep generic multi-stage structure
- docker/docker-compose.yml: Replace term-challenge references with
  generic challenge placeholders
- scripts/build-wasm.sh: Remove term-challenge-specific build lines

BREAKING CHANGE: platform-challenge-sdk-wasm no longer exports
term-challenge-specific types (TermEvaluationParams, CommandRequest,
CommandResult, etc.). Consumers must define these types locally.

Author: echobt