fix(cli): reject malformed numeric options

[GautamSharma99]

Summary

Fixes the CLI numeric-option parsing paths described in #3586.

The agents, environments, and messages subcommands previously used three copies of a parseInt()-based helper. That accepted numeric prefixes such as 10junk, allowed zero and negative pagination values, and silently replaced fully nonnumeric user input with a default. The dream timeout performed a separate partial validation and therefore accepted values such as 30seconds.

This PR routes those options through the existing shared CLI integer-flag owner and makes its parsing strict:

• require the entire trimmed value to contain decimal digits;
• require a JavaScript safe integer greater than zero;
• enforce explicit command bounds;
• use defaults only when an option is omitted;
• return an actionable error and exit code 1 for supplied invalid values.

Bounds

• pagination page sizes (--limit): 1 through 1,000;
• transcript pagination (--max-pages): 1 through 1,000;
• dream reflection timeout (--timeout): 1 through 86,400 seconds.

The CLI help now documents these ranges next to each option.

Implementation details

• Strengthens parsePositiveIntFlag() instead of introducing another numeric parser. Existing callers retain undefined for an omitted flag, while malformed prefixes, fractions, unsafe integers, and out-of-range values now throw a consistent validation error.
• Removes the duplicated parseLimit() implementations from agents, environments, and messages.
• Parses numeric values before settings initialization and backend construction. Invalid input therefore cannot authenticate, initialize settings, or issue an API request.
• Keeps each command's existing omitted-value defaults: agents 20, environments 50, message search 10, message list 20, transcript page size 100, transcript max pages 200, and dream timeout 1,500 seconds.

Tests

Added table-driven coverage for:

• valid complete integers, surrounding whitespace, omitted values, and upper boundaries;
• zero and negative values;
• decimals;
• trailing junk;
• whitespace-only and NaN input;
• integers beyond Number.MAX_SAFE_INTEGER;
• values above each explicit command limit;
• early failure before settings initialization or message/environment backend calls.

Validation performed:

• bun test src/cli/flag-utils.test.ts src/cli/subcommands/agents.test.ts src/cli/subcommands/environments.test.ts src/cli/subcommands/messages.test.ts src/cli/subcommands/dream.test.ts (52 passed)
• bun run check (all 12 checks passed)

Closes #3586