docs: remove false 30-minute idle timeout claim from deployment guides

[examon]

Fixes #2416

What was wrong

The Limitations tables in docs/setup/scaling.md and docs/setup/backend-services.md both claimed the CLI automatically cleans up idle sessions after 30 minutes:

| **30-minute idle timeout** | Sessions without activity are auto-cleaned by the CLI |
| **30-minute idle timeout** | Sessions without activity are auto-cleaned |

There is no such timeout. Idle-session cleanup is opt-in and off by default — sessions are cleaned up for inactivity only when a positive idle timeout is configured on the server, and nothing is configured by default.

The rest of the documentation already says this correctly: docs/features/session-persistence.md states "By default, sessions have no idle timeout", and docs/setup/multi-tenancy.md lists "Not setting sessionIdleTimeoutSeconds" as a common pitfall — a pitfall that only exists because there is no default. Both offending files also contradict themselves within a few lines, recommending hand-rolled periodic cleanup immediately above the row that says the CLI already does it.

Before / after

Both rows, previously:

| **30-minute idle timeout** | Sessions without activity are auto-cleaned by the CLI |

Now, in both files:

| **No idle timeout by default** | Pass `--session-idle-timeout <seconds>` to the CLI server to automatically clean up inactive sessions |

"No idle timeout by default" is a genuine limitation, so the row keeps its place in the table rather than being dropped, and it now points at the setting that actually applies.

Why the server flag rather than sessionIdleTimeoutSeconds

Both of these guides describe connecting to a separately launched CLI server — scaling.md uses cliUrl, backend-services.md uses RuntimeConnection.forUri(...), and both deploy copilot --headless directly. nodejs/src/types.ts documents sessionIdleTimeoutSeconds as "Ignored when connecting to an existing runtime via RuntimeConnection.forUri", and docs/setup/multi-tenancy.md already states the same rule: with forUri(...), configure the idle timeout on the runtime process itself.

Naming the SDK option in these two guides would therefore have sent readers to a setting that is silently dropped in the topology the guides teach. The server flag is also language-neutral, which matters because backend-services.md carries Python, Go, C# and Java examples alongside TypeScript.

Verification

A positive/negative pair, because the CLI logs a line at --log-level debug when it starts its idle-session checker. Tested with CLI 1.0.81-10, the version pinned in nodejs/package-lock.json. Note that --session-idle-timeout does not appear in copilot --help, the same as --port and --headless, which these guides already instruct readers to pass.

mkdir -p logs-a logs-b

# Arm A (control): idle timeout configured
timeout 12 copilot --headless --no-auto-update --port 47321 \
  --log-level debug --log-dir "$PWD/logs-a" --session-idle-timeout 300

# Arm B: the default, option not passed
timeout 12 copilot --headless --no-auto-update --port 47322 \
  --log-level debug --log-dir "$PWD/logs-b"

grep -rh "Started session timeout checker" logs-a/ | wc -l   # -> 1
grep -rh "Started session timeout checker" logs-b/ | wc -l   # -> 0

Arm A logs Started session timeout checker (timeout: 300000ms, check interval: 300000ms); arm B logs no checker line and no idle- or stale-session activity at all. Debug logging is live in both arms — this run produced 22 and 20 [DEBUG] lines, differing by exactly arm A's two checker lines — so arm B's zero is a real absence, not a logging gap. Exact counts vary by version and environment.

Scope was checked repo-wide: these were the only two places claiming a default or automatic idle timeout. The remaining 30-minute references are explicit opt-in examples and recommendations, and are correct as they stand.

Checks

• Documentation only. No source, public API, schema, generated file, or lockfile is touched.
• The docs-validation pipeline extracts fenced code blocks; running it against the tree before and after this change produced byte-identical extraction output (the only difference being the timestamp field), so all five language validation jobs behave exactly as they do on main.
• git diff --check is clean; both rows keep the sibling style of their table (bold first cell, two columns, trailing pipe, no terminal punctuation).

[Copilot]

Pull request overview

Corrects deployment guidance to accurately state that CLI session cleanup is disabled by default.

Changes:

• Replaces the false 30-minute timeout claim.
• Documents the opt-in --session-idle-timeout <seconds> server flag.

Show a summary per file

File	Description
docs/setup/scaling.md	Corrects the scaling limitations table.
docs/setup/backend-services.md	Corrects the backend services limitations table.

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