docs: A features ledger linked from the README

[vpacher]

Refs #1250

What and why

Features are announced in PRs and then disappear in the stream of merges (discussion #1078). This PR adds the features ledger and the rule that keeps it current. It is the first of two PRs for #1250; the second backfills the PRs that merged before the ledger existed.

This PR:

1. Adds docs/features.md. Two sections: Features, by area, and Fixed upstream bugs. One line per feature: what changes for the user, the platforms, the merge date, and the issue and PR links. A follow-up PR adds its link to the existing line. The file explains the format and what counts as user-visible.
2. Seeds it with features that are already merged: the headless changes from Headless and unattended OBS: what already works in the fork #1078 (frontend: Restore --disable-shutdown-check #810, frontend: Migrate pre-31 settings that lack LastVersion #816, frontend: Log output starts refused while outputs are disabled #818, obs-websocket: Save the scene collection after changes #812), the GPU status light (frontend: GPU acceleration status light (green/yellow/red) in the status bar #846) and GPU: x% label (frontend: GPU: x% readout replaces the GPU status light #1025, Linux), 24-bit ALSA capture (linux-alsa: Capture S24_3LE devices at 24 bits #1104), the faster launch on NVIDIA machines (obs-ffmpeg: Probe each VAAPI device once and skip decode-only drivers #1175 to frontend: Probe the GPUs for the GPU status on a thread at launch #1178) and the Qt 6.8 build (frontend: Build with Qt 6.8 (QString::arg) #1017).
3. Adds a Features section to the README that links the ledger.
4. Makes adding the line part of the PR: a rule in coordination.md section 4, a sentence in the obs-worker skill (section 4) and in AGENTS.md, and a checkbox in the PR template. CI, tests, ports with no change in behavior and refactors do not get a line.

The decisions from the claim on #1250: two sections, grouped by area with a merge date per line, plain Markdown for now.

RED → GREEN

• Not applicable: documentation and templates only, no code (like docs: Tell workers how to take a feature issue with open questions #850).
• ci/test_issue_templates.py passes. All of ci/test_*.py through uv run: the same 7 failures as master on macOS, none new.
• Real app: not applicable.

Checklist

• Base is master (or the description starts with a Stacked: note)
• I claimed the issue, or no one else has an open PR for it
• Characterization test (if the unit had none), then RED, then GREEN commits (not applicable: docs only)
• User-visible change: its line in docs/features.md is added or updated (this PR adds the file)
• (Optional) Local gate passed and the pushed head is stamped; otherwise PR CI runs the full suite
• Warning or flag changes (-W..., ratchets): built with scripting ON (a flag change under cmake/ or build-aux/ runs the full builds) and cross-checked with clang and MSVC (not applicable: no flag changes)
• Intentional differences from C listed in the parity test header and rust/README.md (ports) (not applicable: no port)

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