docs: correct the Python Customize Mode section IDs and action list

[examon]

Fixes #2263

The Python README's "Customize Mode" section listed 10 section IDs and described "four string actions". The shipped Python API exposes 12 sections and 5 string actions, so preamble, runtime_instructions and preserve were undiscoverable for anyone following the Python README.

The change

Two lines in python/README.md:

• the "Available section IDs" list now enumerates all 12 members of SystemMessageSection, in declaration order, and notes that identity and tool_instructions are section groups and that preamble targets just the identity preamble;
• the actions sentence now says "five string actions" and includes preserve, with a short gloss of what it is for, and the unknown-section sentence is scoped to the content-bearing actions.

Both additions reuse the wording already in dotnet/README.md, nodejs/README.md, go/README.md and docs/getting-started.md, so the Python README now matches the rest of the documentation rather than introducing a new phrasing.

No code changes.

What the package actually exposes

pip install github-copilot-sdk
python surface.py

surface.py:

import typing

from copilot.session import SectionOverrideAction, SystemMessageSection

section_ids = typing.get_args(SystemMessageSection)
literal_arm = next(
    arg for arg in typing.get_args(SectionOverrideAction)
    if typing.get_origin(arg) is typing.Literal
)
string_actions = typing.get_args(literal_arm)

print(len(section_ids), "section IDs:", list(section_ids))
print(len(string_actions), "string actions:", list(string_actions))

Output:

12 section IDs: ['preamble', 'identity', 'tone', 'tool_efficiency', 'environment_context', 'code_change_rules', 'guidelines', 'safety', 'tool_instructions', 'custom_instructions', 'runtime_instructions', 'last_instructions']
5 string actions: ['replace', 'remove', 'append', 'prepend', 'preserve']

The README previously listed 10 of those 12 section IDs (omitting preamble and runtime_instructions) and 4 of those 5 actions (omitting preserve). After this change it lists all 12 and all 5, in the order shown above.

Checks

python/README.md is the only file changed, so no Python code path is affected.

Run in python/:

• uv run ruff format --check . — passes
• uv run ruff check — All checks passed!
• uv run ty check copilot — exits 0

[Copilot]

Pull request overview

Updates Python documentation to match the shipped system-message customization API.

Changes:

• Documents all 12 section IDs and section-group behavior.
• Documents all five actions, including preserve.
• Clarifies handling of unknown section IDs.

Show a summary per file

File	Description
python/README.md	Corrects Customize Mode sections and actions.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Balanced

[github-actions]

Cross-SDK Consistency Review ✅

This PR modifies only python/README.md — no code changes, no SDK API surface changes.

The documentation correction aligns the Python README with the already-accurate wording in dotnet/README.md, nodejs/README.md, go/README.md, and docs/getting-started.md. No cross-SDK inconsistencies introduced; the change resolves an existing inconsistency between the Python docs and the other SDKs' docs.

No further action needed.

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