feat: add configuration profiles

[bhirsz]

Summary

Implements configuration profiles (#4) — named sets of config overrides
that are selected at runtime with a new builtin --profile option.

[tool.mytool]
loglevel = "INFO"
console = "verbose"

[tool.mytool.profiles.ci]
loglevel = "DEBUG"
console = "dotted"

[tool.mytool.profiles.dev]
inherits = ["ci"]     # merge ci first...
console = "verbose"   # ...then override

$ mytool --profile ci
$ mytool --profile 'ci-*'              # glob
$ mytool --profile ci --profile extra  # multiple, merged in order

Behaviour

• Selection by exact name or fnmatch glob; every pattern must match at
  least one profile, else ConfigDiscoveryError.
• Override, not extend — profile values replace the base (and earlier
  profiles); lists are replaced, not appended.
• inherits (name or list) merges parents first, recursively, with cycle
  detection.
• precedence (int, default 0) orders multiple selected profiles — lower
  applied first, higher wins; ties keep selection order.
• enabled = false skips a directly selected profile (inherited parents
  still contribute).
• Profiles sit in the TOML layer, so CLI args and env vars still win.
• Read from the nearest project config; profiles/inherits/precedence/
  enabled are reserved keys, never treated as options.

Scope (deliberately minimal)

Modeled on a subset of robotcode's profiles. Not included (can follow later):
hidden, detached, default_profiles, extend_profiles, condition
expressions, and env-based profile activation. Value extension from profiles
(append-style extend_*) is intentionally omitted.

Changes

• profiles.py: selection, glob, inherits, precedence, enabled, overlay build.
• base.py: builtin --profile option.
• processor.py: _apply_profiles overlays selected profiles onto the base
  section before mapping; strips the reserved profiles table.
• __init__.py: export build_profile_overlay, select_profiles.
• README.md: new "Profiles" section.
• tests/test_profiles.py: 16 tests.

Closes #4