No-header list mode output

[gadflysu]

Goal

Add a --no-header option for list mode so scripts can consume aps -l output without the table header row.

Problem

aps -l always prints a header before session rows. That is useful for humans, but noisy for shell pipelines: wc -l counts the header as a session and simple grep/text filters can match header text.

Expected Behavior

• aps -l keeps the current header by default.
• aps -l --no-header prints only session rows.
• --no-header only affects list-mode table output.
• --no-header without list mode is accepted but has no visible output effect.

Plan

See docs/agent/plan-issue-37-no-header-list-mode.md.

Acceptance Criteria

• aps -l --no-header omits the header row.
• aps -l still prints the header row.
• --no-header is documented in help output.
• Tests cover flag parsing and list-mode rendering behavior.
• go test ./... passes.
• go build . followed by go install . passes.