feat: v0.8.2 — config templates + agent diagnose + richer error dumps

[dshahnaz]

Summary

The recipe (paste-ready):

```bash
pais agent create --template field-proven \
--name --model openai/gpt-oss-120b --index-id
```

That sets the 5 fields whose null-server-defaults were 502'ing on the user's deployment: `completion_role="assistant"`, `session_max_length=10000`, `session_summarization_strategy="delete_oldest"`, `index_reference_format="structured"`, `chat_system_instruction_mode="system-message"`.

Added

• `--template ` on `pais agent create` (plus 3 new field flags). Templates: `field-proven` / `minimal` / `custom` for agents; `test-suite-bge` / `-arctic` / `code-rag` / `custom` for indexes; `local-files-manual` / `custom` for KBs. Explicit flags always beat seeds.
• `pais templates list [--kind ...]`
• `pais agent dump ` — full server JSON + `__diagnostic` (missing recommended fields, binding mode). Bypasses pydantic defaults so it shows truth on the wire.
• `pais agent diagnose ` — pass/fail check; exits 1 on misses; warns on huge `session_max_length`.

Improved (zero-config)

• chat-errors/*.json now includes `prompt_token_estimate` (no HF needed), `request_body`, `response_headers` (auth stripped), inline `agent_dump`.
• `PaisError.response_headers` carried through the transport.
• HF graceful fallback: `pais.dev.token_budget` falls back to char-based estimation when HuggingFace is unreachable. One `pais.tokenizer.fallback_chars` warning per process. `pais repro` no longer dies on corp networks.

Test plan

• `uv run pytest -q` — 429 passed (+13 new)
• `uv run ruff check && uv run ruff format --check && uv run mypy src` — clean
• New tests:
  • `test_templates.py` (9): template structure + apply_template + `pais templates list` + `--template field-proven` produces expected wire body + explicit flag beats template + unknown-template error
  • `test_agent_dump_and_diagnose.py` (4): dump shows missing fields, diagnose minimal exits 1, diagnose field-proven exits 0, WARN on session_max_length=100000
• CI green on 3.10 / 3.11 / 3.12

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