feat(config): support env-var-only startup without config file

[canyugs]

Description

Allow OpenAB to be fully configured through environment variables alone,
without requiring a config.toml file at startup. Every setting currently
readable from the config file should have a corresponding OPENAB_* env var
that OpenAB reads when no config file is present (or when
OPENAB_CONFIG_FROM_ENV=1 is set).

This affects the core config loader (src/config.rs) and the Helm chart
(charts/openab).

Use Case

Kubernetes operators today must bake a config.toml into a ConfigMap or mount
it as a Secret volume, then keep it in sync with Helm values. This creates two
sources of truth.

As a platform operator, I want to configure OpenAB entirely via environment
variables so that:

• There is a single source of truth (Helm release values / Kubernetes Secrets).
• Secrets (bot tokens, API keys) are never written to a ConfigMap as plaintext.
• The deployment follows 12-factor app principles and works with standard secret
  managers (Vault, External Secrets Operator, AWS Secrets Manager).

As a developer or CI runner, env-var-only mode also makes testing significantly
easier — spin up an instance with a few exported variables, no config file to
create, template, or clean up between test runs.

Proposed Solution

Define a flat set of OPENAB_* env vars that mirror the TOML structure:

Env var	TOML equivalent
OPENAB_DISCORD_BOT_TOKEN	[discord] bot_token
OPENAB_DISCORD_ALLOWED_CHANNELS	[discord] allowed_channels (comma-separated)
OPENAB_DISCORD_ALLOW_BOT_MESSAGES	[discord] allow_bot_messages
OPENAB_SLACK_BOT_TOKEN	[slack] bot_token
OPENAB_SLACK_APP_TOKEN	[slack] app_token
OPENAB_AGENT_COMMAND	[agent] command
OPENAB_AGENT_ARGS	[agent] args (space-separated or JSON array)
OPENAB_AGENT_WORKING_DIR	[agent] working_dir
OPENAB_POOL_MAX_SESSIONS	[pool] max_sessions
OPENAB_POOL_SESSION_TTL_HOURS	[pool] session_ttl_hours
…	…

In src/config.rs, when --config is not provided and config.toml does not
exist, fall back to building a Config struct from these env vars. The Helm
chart would expose a corresponding envVars: section in values.yaml and
stop generating a ConfigMap when env-var mode is active.

Prior Art

OpenClaw — supports layered env sources (process env → .env →
~/.openclaw/.env → config env block) and full ${VAR_NAME} substitution
in all config strings. Does not yet support a fully config-file-free startup,
but the env-layer precedence model is directly applicable.
Ref: docs/help/environment.md, docs/gateway/configuration-reference.md.

Hermes Agent — defines a comprehensive HERMES_* env var namespace
covering every feature (provider, model, gateway, session, etc.), documented in
website/docs/reference/environment-variables.md. The agent can be started
with no config file at all. This is the closest prior art to the proposed
feature.

Related Issues

• docs: add Helm values reference with undocumented options #163 — docs for undocumented Helm values (complementary; env-var docs would extend this)
• Add pod/deployment extensibility controls to Helm chart #397 — Helm pod extensibility (complementary; env-var config would be one more extensibility surface)
• feat(chart): expose maxBotTurns for Slack and Discord adapters #609 — expose maxBotTurns via Helm (same pattern, single field; this proposes a systematic approach)

[CHC-Agent]

Issue at a Glance

Current config loading:
  main.rs L96 ──► config.toml (required)
       │
       ▼
  config.rs::load_config()
       │
       ▼
  expand_env_vars("${VAR}") ──► substitutes values inside TOML
       │
       ▼
  toml::from_str() ──► Config struct

Proposed:
  main.rs ──► --config flag provided?
       │                │
      YES              NO
       │                │
       ▼                ▼
  load_config()    OPENAB_CONFIG_FROM_ENV=1?
                        │
                       YES
                        │
                        ▼
                   build Config from OPENAB_* env vars
                   (no config.toml needed)

Hi @canyugs,

Well-scoped proposal with clear use cases. The current expand_env_vars() in config.rs handles ${VAR} substitution within TOML values, but there is no path to build a Config struct purely from environment variables.

Related:

• docs: add Helm values reference with undocumented options #163 — Helm values reference docs (OPEN, complementary)
• Add pod/deployment extensibility controls to Helm chart #397 — Helm pod extensibility (OPEN, complementary)
• feat(chart): expose maxBotTurns for Slack and Discord adapters #609 — expose maxBotTurns via Helm (OPEN, incomplete — a subset of this proposal)

The scope touches config.rs (env var → Config builder) and the Helm chart (charts/openab). Prior art from OpenClaw (layered env sources) and Hermes Agent (HERMES_* namespace) provides good reference implementations. Needs maintainer discussion on implementation strategy and phasing.