fix(sessions): derive default capacity from RAM

[defangdevs]

Why

Issue #662 specified a default budget of roughly one session per GiB, including four sessions on the default 4 GiB boxes. PR #668 changed the implementation plan to a static default of four to avoid kernel memory reservations rounding a nominal 4 GiB box down to three. That made large boxes keep the four-session ceiling; this 62.6 GiB box still reported 4 instead of roughly 60.

What changed

• Make an unset sessionLimit automatic and derive it from /proc/meminfo MemTotal.
• Round up to the next GiB so nominal 4 GiB deployments still receive four slots; enforce a minimum of one.
• Preserve an explicit positive-integer sessionLimit override.
• Fail closed if the automatic RAM probe is malformed or unavailable.
• Update CLI/help text, module/native renderers, generated output, and golden fixtures.
• Add unit coverage for automatic boundaries, missing config, explicit override, and malformed inputs.

The default on the current 62.6 GiB host computes to 63 sessions.

User-visible and migration impact

Deployments without an explicit sessionLimit move from a fixed limit of 4 to approximately one slot per GiB of physical RAM. Deployments that intentionally rely on four can set sessionLimit = 4; existing explicit settings are unchanged. This remains admission control, not a per-session memory guarantee.

Security / infrastructure impact

No IAM, network, AWS resource, image-format, or direct cloud-cost changes. The larger automatic limit allows more local agent processes on larger hosts, proportional to their RAM. No data migration is required.

Validation

• nix run .#assemble
• nix run .#update-native-config
• nix run .#update-golden
• python3 tests/test-session-capacity.py — 16 passed
• python3 tests/test_agentbox.py — 150 passed, 1 skipped
• nix build -L --keep-going --max-jobs 4 --cores 1 --no-link .#ci-native
• nix build -L --no-link .#checks.x86_64-linux.session-limit
• git diff --check

Follow-up to #662 and #668.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 0e85416 and 0289f46.

📒 Files selected for processing (25)

• README.md
• bin/agentbox
• flake.nix
• modules/agent-box.nix
• modules/agent-box.nix.in
• modules/src/default-agents-webhook.md
• modules/src/default-agents.md
• modules/src/lib/session-capacity.py
• modules/src/session-cli.sh
• modules/src/webhook-cli.sh
• tests/golden/DUPLICATES
• tests/golden/vm/etc/agent-box-guides/AGENTS.agent.md
• tests/golden/vm/etc/agent-box/session-limit
• tests/golden/vm/payloads/agent-box-session-capacity/bin/agent-box-session-capacity
• tests/golden/vm/payloads/agent-box-session/bin/agent-box-session
• tests/golden/web/etc/agent-box-guides/AGENTS.agent.md
• tests/golden/web/payloads/agent-box-settings/bin/agent-box-settings
• tests/golden/web/payloads/agent-box-webhook/bin/agent-box-webhook
• tests/native/config.json
• tests/native/config.yaml
• tests/native/expected/etc/agent-box-guides/AGENTS.agent.md
• tests/native/expected/etc/agent-box-guides/AGENTS.robot.md
• tests/native/expected/etc/agent-box/session-limit
• tests/test-session-capacity.py
• tests/test_agentbox.py

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